Commit 0e9b5cd6 "migration: introduce UFFD-WP low-level interface
helpers" added util/userfaultfd.c without covering it in MAINTAINERS.
Add it to section "Migration".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230119091545.3116376-2-armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>