fat: remove redundant sys_tz declaration

sys_tz is already declared extern struct in include/linux/time.h

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>