MFC: rc.d/rctl: unbreak for distinct /usr filesystem

Both rctl and used xargs utility live in /usr/bin
so add REQUIRE: FILESYSTEMS

This trivial change to stable/12 violates "3 days MFC policy"
because of imminent upcoming 12.3-RELESE deadline.

Approved by: re (cperciva)
Reported by: Peter <pmc@citylink.dinoex.sub.org>

(cherry picked from commit 0c54fe172ad365e7e60d6249484a7579c18b7d2d)