GCC4.6: Squash warning in vmt.c

vmt.c: In function ‘ubi_free_volume’:
vmt.c:681:6: warning: variable ‘err’ set but not used
[-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>