Add empty constructor to workaround gcc/clang bug (#31780)

Some versions of gcc/clang have a bug that fail to compile
with an error message without a constructor definition.
Adding a no-op constructor to avoid this issue.

src/core/ext/xds/xds_listener.h

@@ -79,6 +79,8 @@ struct XdsListenerResource : public XdsResourceType::ResourceData {
   };
 
   struct DownstreamTlsContext {
+    DownstreamTlsContext() {}
+
     CommonTlsContext common_tls_context;
     bool require_client_certificate = false;