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>

pkgs/top-level/python-packages.nix

@@ -4524,6 +4524,10 @@ pythonPackages = modules // import ./python-packages-generated.nix {
   protobuf = buildPythonPackage rec {
     inherit (pkgs.protobuf) name src;
 
+    buildPhase = ''
+      python setup.py build
+    '';
+
     propagatedBuildInputs = [pkgs.protobuf];
     sourceRoot = "${name}/python";