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
diff --git a/third_party/protobuf-c b/third_party/protobuf-c
new file mode 160000
index 0000000..006d69b
--- /dev/null
+++ b/third_party/protobuf-c
@@ -0,0 +1 @@
+Subproject commit 006d69bd8481738323c58af5cd4c8477d09eb9f5