Fix BUILD.gn file

BUILD.gn

@@ -1014,6 +1014,7 @@ config("grpc_config") {
         "include/grpcpp/alarm.h",
         "include/grpcpp/alarm_impl.h",
         "include/grpcpp/channel.h",
+        "include/grpcpp/channel_impl.h",
         "include/grpcpp/client_context.h",
         "include/grpcpp/completion_queue.h",
         "include/grpcpp/create_channel.h",