python-protobuf: fix legal info

It uses the same tarball as protobuf hence suffers the same problem as
commit f700463c. Fixes:
http://autobuild.buildroot.net/results/428/4282281e5582b0da8ed3ff44f4d6d86f716ea333/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>