libselinux: change to github location

libselinux: change to github location

Fixes:
http://autobuild.buildroot.net/results/4a7/4a7b71f1471b889abf278882a7735475dd95451b/
http://autobuild.buildroot.net/results/bdf/bdf621c5a893bce51db95f4a8f2814c14d2f6d49/
http://autobuild.buildroot.net/results/7a8/7a8426f60a7377e3b08e6502f4e0b2c738002a79/
http://autobuild.buildroot.net/results/d90/d90e40f7fb6e1a7d429454d04b9f5f69826a842e/

And many more.

The old location doesn't seem active any more. Even though this github URL
looks a bit odd, it is linked from
https://github.com/SELinuxProject/selinux/wiki/Releases

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>