Fix build error with !CONFIG_MIGRATION

Fix build error with !CONFIG_MIGRATION

migrate_huge_page_move_mapping() is declared as "extern int ..."
in include/linux/migrate.h for !CONFIG_MIGRATION,
which causes the build error like below:

  mm/mprotect.o: In function `migrate_huge_page_move_mapping':
  mprotect.c:(.text+0x0): multiple definition of `migrate_huge_page_move_mapping'
  mm/shmem.o:shmem.c:(.text+0x0): first defined here
  mm/rmap.o: In function `migrate_huge_page_move_mapping':
  rmap.c:(.text+0x0): multiple definition of `migrate_huge_page_move_mapping'
  mm/shmem.o:shmem.c:(.text+0x0): first defined here

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>