Remove cast when using BoringSSL.

BoringSSL defines RSA_size as returning an unsigned, as one would hope.
No cast is needed in that case.

Change-Id: Ifc596af987a6abce40f685d66adb63aee4709666
diff --git a/rsa_operation.cpp b/rsa_operation.cpp
index 4eef5c1..a013b78 100644
--- a/rsa_operation.cpp
+++ b/rsa_operation.cpp
@@ -60,9 +60,14 @@
 }
 
 keymaster_error_t RsaVerifyOperation::Finish(const Buffer& signature, Buffer* /* output */) {
-
+#if defined(OPENSSL_IS_BORINGSSL)
+    if (data_.available_read() != RSA_size(rsa_key_))
+        return KM_ERROR_INVALID_INPUT_LENGTH;
+#else
     if ((int)data_.available_read() != RSA_size(rsa_key_))
         return KM_ERROR_INVALID_INPUT_LENGTH;
+#endif
+
     if (data_.available_read() != signature.available_read())
         return KM_ERROR_VERIFICATION_FAILED;