message: Fix build with google_grpc=disabled
diff: @@ -38,6 +38,8 @@ AsyncClientManagerImpl::AsyncClientManagerImpl(Upstream::ClusterManager& cm,
  google_tls_slot_ = tls.allocateSlot();
  google_tls_slot_->set(
  [&api](Event::Dispatcher&) { return std::make_shared<GoogleAsyncClientThreadLocal>(api); });
  +#else
  + UNREFERENCED_PARAMETER(api);
  #endif
  }
  