stand: fix build userboot without zfs

Fix regression in building userboot -DWITHOUT_LOADER_ZFS

Fixes: e307eb94ae520
MFC After: 3 days
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/665
Approved by: re@ (cperciva)

(cherry picked from commit 4d59545d0cacb5881e2515eecce15e8478f07276)
(cherry picked from commit 3464fc5f2a3376b4cd5612f1ee0986072b4a65f3)