Add protoc include to avoid descriptor.proto with newer gogoprotobuf

See gogoprotobuf commits a5a88af658a4378ad4a64419850ec2669b1ef253
and 2e6b891c6f1eee07d068d26a9d61816d42111bad.
Merge pull request #3 from tv42/master

Add protoc include to avoid descriptor.proto with newer gogoprotobuf