maintner/maintpb: use google.protobuf.Timestamp

Add $GOPATH/src to the --proto_path so we can import the google
Timestamp definition instead of redefining a copy in our tree.

Change-Id: I09c3554fb025a32c9db4892b702ee2bedb3c0e53
Reviewed-on: https://go-review.googlesource.com/36899
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed