Making fs/minix/minix.h double including safe

I happened to find that fs/minix/minix.h doesn't guard double include.

Yes, I know this never cause something destructive because this is
self-evidence that no source file includes minix.h twice, but I think
fixing this is better than disregarding it.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>