sepolicy: Add ostree_sepolicy_set_null_log

sepolicy: Add ostree_sepolicy_set_null_log

Let's quiet the output of libselinux by default, all we
get is the pointless "regex version mismatch" warnings.
This is what e.g. systemd does.

This is exposed via the library; our binary opts in,
then we expect higher level tools like e.g. bootc to
do this too.

Signed-off-by: Colin Walters <walters@verbum.org>