libostree: write selinux xattr when on non-selinux systems

libostree: write selinux xattr when on non-selinux systems

Currently when writing data for selinux systems on a non-selinux
system there will be no labels. This is because
`ostree_sepolicy_setfscreatecon()` just returns TRUE on non-selinux
systems and xattr writing for `security.seliux` is filtered out.

This patches uses the suggestion of Colin Walters (thanks!) from
https://github.com/ostreedev/ostree/issues/2804 and detects if
the host has selinux enabled and if not just skips filtering the
xattrs for selinux.