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

Author: Lexi Winter

release/packages/ucl/rc.ucl

@@ -19,6 +19,9 @@
 deps {
 	"runtime": {
 		version = "${VERSION}"
-	}
+	},
+	# /etc/rc.d/var_run depends on mtree.
+	"mtree": {
+		version = "${VERSION}"
+	},
 }
-