Restored .gitmodules as per review request at #357 (review).

shiniwat · shiniwat · commit 819e36676844 · 2021-01-14T13:37:47.000+09:00
diff --git a/.gitmodules b/.gitmodules
@@ -1,3 +1,4 @@
 [submodule "lib/rpc_spec"]
 	path = lib/rpc_spec
 	url = https://github.com/smartdevicelink/rpc_spec.git
+	branch = master
