libselinux: add patch to fix build with gcc < 4.7

This commit adds a patch from Hollis Blanchard on libselinux to fix
build on host machines that have gcc < 4.7.

Fixes:

  http://autobuild.buildroot.net/results/a82bb0c0b22ff24263ad7a7d165b21c0df7b3b1d/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>