libselinux: set an appropriate errno in booleans.c

Fixes:
$ mkdir booleans
$ sudo mount --bind ./booleans /sys/fs/selinux/booleans
$ sudo getsebool -a
getsebool:  Unable to get boolean names:  Success

Signed-off-by: Petr Lautrbach <plautrba@redhat.com>
1 file changed