Fix open-telemetry#2545

Author: marcalff

exporters/otlp/src/otlp_grpc_client.cc

@@ -296,7 +296,7 @@ std::shared_ptr<grpc::Channel> OtlpGrpcClient::MakeChannel(const OtlpGrpcClientO
     grpc::SslCredentialsOptions ssl_opts;
     ssl_opts.pem_root_certs = GetFileContentsOrInMemoryContents(
         options.ssl_credentials_cacert_path, options.ssl_credentials_cacert_as_string);
-#if ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW
+#ifdef ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW
     ssl_opts.pem_private_key = GetFileContentsOrInMemoryContents(options.ssl_client_key_path,
                                                                  options.ssl_client_key_string);
     ssl_opts.pem_cert_chain  = GetFileContentsOrInMemoryContents(options.ssl_client_cert_path,