libsepol: fix static-only library build

Add a patch that improves the libsepol build system to support
static-only library build.

Fixes:

  http://autobuild.buildroot.org/results/30c/30cb919d2bbf51bfdf0a83c618124d927d046a61/build-end.log



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