zfs: Disable -Wunused-variable for environ in main in zfs_main.c.

There is an extern declaration of the global variable environ (which
has no portable header that defines it) that is unused on FreeBSD due
to zfs_setproctitle_init() being a stub.

Reviewed by: emaste
Reported by: GCC
Differential Revision: https://reviews.freebsd.org/D38231