ifdef out EVP_R_PRIVATE_KEY_DECODE_ERROR

EVP_R_PRIVATE_KEY_DECODE_ERROR was added in OpenSSL 1.0.0 so ifdef out
for the benefit of OpenSSL versions prior to that.
diff --git a/sshkey.c b/sshkey.c
index 0567592..31710e5 100644
--- a/sshkey.c
+++ b/sshkey.c
@@ -3812,7 +3812,9 @@
 				goto out;
 			case EVP_R_BN_DECODE_ERROR:
 			case EVP_R_DECODE_ERROR:
+#ifdef EVP_R_PRIVATE_KEY_DECODE_ERROR
 			case EVP_R_PRIVATE_KEY_DECODE_ERROR:
+#endif
 				r = SSH_ERR_INVALID_FORMAT;
 				goto out;
 			default: