ksm: fix m68k build: only NUMA needs pfn_to_nid

A CONFIG_DISCONTIGMEM=y m68k config gave

  mm/ksm.c: In function `get_kpfn_nid':
  mm/ksm.c:492: error: implicit declaration of function `pfn_to_nid'

linux/mmzone.h declares it for CONFIG_SPARSEMEM and CONFIG_FLATMEM, but
expects the arch's asm/mmzone.h to declare it for CONFIG_DISCONTIGMEM
(see arch/mips/include/asm/mmzone.h for example).

Or perhaps it is only expected when CONFIG_NUMA=y: too much of a maze,
and m68k got away without it so far, so fix the build in mm/ksm.c.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Petr Holasek <pholasek@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>