add protobuf-c module
diff --git a/.gitmodules b/.gitmodules
index 46e4dc7..e98f319 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -4,3 +4,6 @@
 [submodule "third_party/grpc"]
 	path = third_party/grpc
 	url = https://github.com/grpc/grpc.git
+[submodule "third_party/protobuf-c"]
+	path = third_party/protobuf-c
+	url = https://github.com/protobuf-c/protobuf-c.git