libselinux: add patch to fix Blackfin build issue

Fixes:

  http://autobuild.buildroot.org/results/165/165a227a0a8ecd4cb3f96761aacdf90ae974fea7/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>