Fix compile error on nommu for is_swap_pte

  CC      mm/vmscan.o
In file included from
/home/bunk/linux/kernel-2.6/git/linux-2.6/mm/vmscan.c:44:
/home/bunk/linux/kernel-2.6/git/linux-2.6/include/linux/swapops.h: In function 'is_swap_pte':
/home/bunk/linux/kernel-2.6/git/linux-2.6/include/linux/swapops.h:48: error: implicit declaration of function 'pte_none'
/home/bunk/linux/kernel-2.6/git/linux-2.6/include/linux/swapops.h:48: error: implicit declaration of function 'pte_present'

Does it ever make sense to ask "is this pte a swap entry?" on a machine
with no MMU?  Presumably this also means it has no ptes too, right?  In
which case, it's better to comment the whole function out.  Then when
someone tries to ask the above meaningless question, they get a compile
error rather than a meaningless answer.

Signed-off-by: Matt Mackall <mpm@selenic.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Reported-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>