mtree: Add missing libxo/encoder path

Reviewed by: imp
Approved by: emaste (mentor)
Fixes: d1a0d267b78b ("Upgrade libxo to 0.4.5.")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48836

(cherry picked from commit d10deaea12adb73a698d907a148f081c402d5dd6)