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

cddl/sbin/zfs/Makefile

@@ -77,3 +77,5 @@ CFLAGS+= \
 LIBADD=	jail avl nvpair geom  uutil zfs_core spl tpool zutil zfs m crypto
 LDADD+=	-pthread
 .include <bsd.prog.mk>
+
+CWARNFLAGS.zfs_main.c+=	-Wno-unused-variable