third_party: Fix mbedtls gn build issue

gn variable |mbedtls_sources| should be guarded by
dir_pw_third_party_mbedtls != "" condition. Otherwise gn complains
that assignment has no effect.

Change-Id: I177a3f5dd2df298136c7ed1fbde2b96d037dfe6b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/50741
Reviewed-by: Varun Sharma <vars@google.com>
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
diff --git a/third_party/mbedtls/BUILD.gn b/third_party/mbedtls/BUILD.gn
index 150fe39..49ea3d5 100644
--- a/third_party/mbedtls/BUILD.gn
+++ b/third_party/mbedtls/BUILD.gn
@@ -25,97 +25,97 @@
   pw_third_party_mbedtls_CONFIG = "default"
 }
 
-# The list currently includes all source files for build.
-mbedtls_sources = [
-  "library/aes.c",
-  "library/aesni.c",
-  "library/arc4.c",
-  "library/aria.c",
-  "library/asn1parse.c",
-  "library/asn1write.c",
-  "library/base64.c",
-  "library/bignum.c",
-  "library/blowfish.c",
-  "library/camellia.c",
-  "library/ccm.c",
-  "library/certs.c",
-  "library/chacha20.c",
-  "library/chachapoly.c",
-  "library/cipher.c",
-  "library/cipher_wrap.c",
-  "library/cmac.c",
-  "library/ctr_drbg.c",
-  "library/debug.c",
-  "library/des.c",
-  "library/dhm.c",
-  "library/ecdh.c",
-  "library/ecdsa.c",
-  "library/ecjpake.c",
-  "library/ecp.c",
-  "library/ecp_curves.c",
-  "library/entropy.c",
-  "library/entropy_poll.c",
-  "library/error.c",
-  "library/gcm.c",
-  "library/havege.c",
-  "library/hkdf.c",
-  "library/hmac_drbg.c",
-  "library/md.c",
-  "library/md2.c",
-  "library/md4.c",
-  "library/md5.c",
-  "library/memory_buffer_alloc.c",
-  "library/net_sockets.c",
-  "library/nist_kw.c",
-  "library/oid.c",
-  "library/padlock.c",
-  "library/pem.c",
-  "library/pk.c",
-  "library/pk_wrap.c",
-  "library/pkcs11.c",
-  "library/pkcs12.c",
-  "library/pkcs5.c",
-  "library/pkparse.c",
-  "library/pkwrite.c",
-  "library/platform.c",
-  "library/platform_util.c",
-  "library/poly1305.c",
-  "library/psa_crypto.c",
-  "library/psa_crypto_driver_wrappers.c",
-  "library/psa_crypto_se.c",
-  "library/psa_crypto_slot_management.c",
-  "library/psa_crypto_storage.c",
-  "library/psa_its_file.c",
-  "library/ripemd160.c",
-  "library/rsa.c",
-  "library/rsa_internal.c",
-  "library/sha1.c",
-  "library/sha256.c",
-  "library/sha512.c",
-  "library/ssl_cache.c",
-  "library/ssl_ciphersuites.c",
-  "library/ssl_cli.c",
-  "library/ssl_cookie.c",
-  "library/ssl_msg.c",
-  "library/ssl_srv.c",
-  "library/ssl_ticket.c",
-  "library/ssl_tls.c",
-  "library/ssl_tls13_keys.c",
-  "library/threading.c",
-  "library/timing.c",
-  "library/version.c",
-  "library/version_features.c",
-  "library/x509.c",
-  "library/x509_create.c",
-  "library/x509_crl.c",
-  "library/x509_crt.c",
-  "library/x509_csr.c",
-  "library/x509write_crt.c",
-  "library/x509write_csr.c",
-  "library/xtea.c",
-]
-
 if (dir_pw_third_party_mbedtls != "") {
+  # The list currently includes all source files for build.
+  mbedtls_sources = [
+    "library/aes.c",
+    "library/aesni.c",
+    "library/arc4.c",
+    "library/aria.c",
+    "library/asn1parse.c",
+    "library/asn1write.c",
+    "library/base64.c",
+    "library/bignum.c",
+    "library/blowfish.c",
+    "library/camellia.c",
+    "library/ccm.c",
+    "library/certs.c",
+    "library/chacha20.c",
+    "library/chachapoly.c",
+    "library/cipher.c",
+    "library/cipher_wrap.c",
+    "library/cmac.c",
+    "library/ctr_drbg.c",
+    "library/debug.c",
+    "library/des.c",
+    "library/dhm.c",
+    "library/ecdh.c",
+    "library/ecdsa.c",
+    "library/ecjpake.c",
+    "library/ecp.c",
+    "library/ecp_curves.c",
+    "library/entropy.c",
+    "library/entropy_poll.c",
+    "library/error.c",
+    "library/gcm.c",
+    "library/havege.c",
+    "library/hkdf.c",
+    "library/hmac_drbg.c",
+    "library/md.c",
+    "library/md2.c",
+    "library/md4.c",
+    "library/md5.c",
+    "library/memory_buffer_alloc.c",
+    "library/net_sockets.c",
+    "library/nist_kw.c",
+    "library/oid.c",
+    "library/padlock.c",
+    "library/pem.c",
+    "library/pk.c",
+    "library/pk_wrap.c",
+    "library/pkcs11.c",
+    "library/pkcs12.c",
+    "library/pkcs5.c",
+    "library/pkparse.c",
+    "library/pkwrite.c",
+    "library/platform.c",
+    "library/platform_util.c",
+    "library/poly1305.c",
+    "library/psa_crypto.c",
+    "library/psa_crypto_driver_wrappers.c",
+    "library/psa_crypto_se.c",
+    "library/psa_crypto_slot_management.c",
+    "library/psa_crypto_storage.c",
+    "library/psa_its_file.c",
+    "library/ripemd160.c",
+    "library/rsa.c",
+    "library/rsa_internal.c",
+    "library/sha1.c",
+    "library/sha256.c",
+    "library/sha512.c",
+    "library/ssl_cache.c",
+    "library/ssl_ciphersuites.c",
+    "library/ssl_cli.c",
+    "library/ssl_cookie.c",
+    "library/ssl_msg.c",
+    "library/ssl_srv.c",
+    "library/ssl_ticket.c",
+    "library/ssl_tls.c",
+    "library/ssl_tls13_keys.c",
+    "library/threading.c",
+    "library/timing.c",
+    "library/version.c",
+    "library/version_features.c",
+    "library/x509.c",
+    "library/x509_create.c",
+    "library/x509_crl.c",
+    "library/x509_crt.c",
+    "library/x509_csr.c",
+    "library/x509write_crt.c",
+    "library/x509write_csr.c",
+    "library/xtea.c",
+  ]
+
   mbedtls_configs = [
     {
       name = "default"