mtree: Add missing flua/freebsd/sys entry

Reviewed by: imp
Approved by: emaste (mentor)
Fixes: de1d7d7b87cf ("flua: add freebsd module implementing kldload/kldunload")
Differential Revision: https://reviews.freebsd.org/D48838

(cherry picked from commit c587e4c7e4c28354634b4e13f30743ccdd3fbf91)