python-protobuf: Fix generating *_pb2.py files.

This only happens during setup.py build and generates the corresponding
_pb2.py files from the .proto files, as they're no longer included in
the source distribution starting from 2.5.0.

Thanks to @iElectric for reporting this build failure:

http://hydra.nixos.org/build/6897112

Signed-off-by: aszlig <aszlig@redmoonstudios.org>