fstyp(8): define HAVE_ZFS macro when built with zfs

The HAVE_ZFS macro was dropped from the Makefile during the OpenZFS
merge, bring it back.

While here, drop unused WARNS setting.

PR: 255616
Reported by: Michael Büker <freebsd@michael-bueker.de>
Submitted by: Michael Büker <freebsd@michael-bueker.de>
Fixes: 9e5787d2284e187abb5b654d924394a65772e004
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D30221

(cherry picked from commit 91f251b2ab3ade5de703756873142bfae2573753)