php-geoip: fix legal info

The LICENSE file is gone, so use the lone .c source fle. Fixes:
http://autobuild.buildroot.net/results/acd/acd763e7eefb8be2bbd0bf973bb7855fceee7377/



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