The HAVE_ZFS macro was dropped from the Makefile during the OpenZFS
merge, bring it back.
PR: 255616
Reported by: Michael Büker <freebsd@michael-bueker.de>
Submitted by: Michael Büker <freebsd@michael-bueker.de>
Fixes: 9e5787d2284e187abb5b654d924394a65772e004