protobuf: fix legal-info

The file changed from COPYING.txt to LICENSE, fixes:
http://autobuild.buildroot.net/results/c40/c40e9b6030584a4fd8b3e46f78f6d0702d1dc11e/



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