mtree: Add missing flua entry

[jlduran]

Summary

This is a direct commit to the stable/14 and stable/13 branch.

Fixes: bceabe277e12 ("flua: initial support for "require" in the base system")

Test Plan

When trying to cherry-pick c587e4c7e4c2 ("mtree: Add missing flua/freebsd/sys entry"), I realized it should have been done in two steps, one for this fix first, and then the subdirectories.
I do not know if I should just cherry-pick and remove the subdirectories, or this "direct commit".