Ignore unused variable warning in gRPC.

Change-Id: I80c94cb2b3d5606d61cb7520687bd0b0bbb02ec5
diff --git a/third_party/grpc/src/cpp/client/create_channel.cc b/third_party/grpc/src/cpp/client/create_channel.cc
index fdaa28f..bb8b201 100644
--- a/third_party/grpc/src/cpp/client/create_channel.cc
+++ b/third_party/grpc/src/cpp/client/create_channel.cc
@@ -54,7 +54,7 @@
     const grpc::string& target,
     const std::shared_ptr<ChannelCredentials>& creds,
     const ChannelArguments& args) {
-  internal::GrpcLibrary
+  __attribute__((unused)) internal::GrpcLibrary
       init_lib;  // We need to call init in case of a bad creds.
   ChannelArguments cp_args = args;
   std::ostringstream user_agent_prefix;