Add missing dep to the get_model_metadata_proto_py_pb2 build target.
Add missing dep to the get_model_metadata_proto_py_pb2 build target.

This caused an issue when linking as a submodule - https://github.com/tensorflow/serving/issues/328
Change: 148122704

tensorflow/serving,tensorflow/serving,avloss/serving,addappio/serving,avloss/serving,penguin138/serving,tensorflow/serving,avloss/serving,penguin138/serving,sreddybr3/serving,avloss/serving,sreddybr3/serving,addappio/serving,tensorflow/serving,penguin138/serving,sreddybr3/serving,penguin138/serving,addappio/serving,addappio/serving,sreddybr3/serving