Fix BUILD.gn files

BUILD.gn

@@ -1004,6 +1004,7 @@ config("grpc_config") {
         "include/grpcpp/client_context.h",
         "include/grpcpp/completion_queue.h",
         "include/grpcpp/create_channel.h",
+        "include/grpcpp/create_channel_impl.h",
         "include/grpcpp/create_channel_posix.h",
         "include/grpcpp/ext/health_check_service_server_builder_option.h",
         "include/grpcpp/generic/async_generic_service.h",