package/libselinux: fix the selinuxfs mount point

package/libselinux: fix the selinuxfs mount point

For 9 years the recommended mount point for selinuxfs has been
/sys/fs/selinux, as stated in Linux kernel commit 7a627e3b9a2b:

"""
For selinuxfs, this mount point should be in /sys/fs/selinux/
"""

As other projects follow this convention, not doing so result in
potential issues. One of them is the refpolicy not correctly labelling
and supporting the mount point.

Fix this by using /sys/fs/selinux as of now in Buildroot.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>