packages: Add dependency from rc to mtree

/etc/rc.d/var_run uses mtree, which is in the devel set and isn't
installed as part of either minimal or optional, so add a manual
dependency.

Reviewed by: des, bapt, emaste
Differential Revision: https://reviews.freebsd.org/D54540
Sponsored by: https://www.patreon.com/bsdivy