package/python-protobuf: eliminate unneeded build-time dependencies

This was causing unexpected HTTP requests by the setup.py script for
python-protobuf. These "dependencies" aren't actually required for a successful
build, and are not staged into the target install directory.

Fixes:

  http://autobuild.buildroot.org/results/e44/e44694f30e39266491a1040e284e504d6d37ef9e/



Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>