add protobuf submodule
diff --git a/.gitmodules b/.gitmodules
index e98f319..411fab3 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -7,3 +7,6 @@
 [submodule "third_party/protobuf-c"]
 	path = third_party/protobuf-c
 	url = https://github.com/protobuf-c/protobuf-c.git
+[submodule "third_party/protobuf"]
+	path = third_party/protobuf
+	url = https://github.com/google/protobuf.git