ia64: fix build errors by exporting paddr_to_nid()

Fix build errors on ia64 when DISCONTIGMEM=y and NUMA=y by
exporting paddr_to_nid().

Fixes these build errors:

ERROR: "paddr_to_nid" [sound/core/snd-pcm.ko] undefined!
ERROR: "paddr_to_nid" [net/sunrpc/sunrpc.ko] undefined!
ERROR: "paddr_to_nid" [fs/cifs/cifs.ko] undefined!
ERROR: "paddr_to_nid" [drivers/video/fbdev/core/fb.ko] undefined!
ERROR: "paddr_to_nid" [drivers/usb/mon/usbmon.ko] undefined!
ERROR: "paddr_to_nid" [drivers/usb/core/usbcore.ko] undefined!
ERROR: "paddr_to_nid" [drivers/md/raid1.ko] undefined!
ERROR: "paddr_to_nid" [drivers/md/dm-mod.ko] undefined!
ERROR: "paddr_to_nid" [drivers/md/dm-crypt.ko] undefined!
ERROR: "paddr_to_nid" [drivers/md/dm-bufio.ko] undefined!
ERROR: "paddr_to_nid" [drivers/ide/ide-core.ko] undefined!
ERROR: "paddr_to_nid" [drivers/ide/ide-cd_mod.ko] undefined!
ERROR: "paddr_to_nid" [drivers/gpu/drm/drm.ko] undefined!
ERROR: "paddr_to_nid" [drivers/char/agp/agpgart.ko] undefined!
ERROR: "paddr_to_nid" [drivers/block/nbd.ko] undefined!
ERROR: "paddr_to_nid" [drivers/block/loop.ko] undefined!
ERROR: "paddr_to_nid" [drivers/block/brd.ko] undefined!
ERROR: "paddr_to_nid" [crypto/ccm.ko] undefined!

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: linux-ia64@vger.kernel.org
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>