Impact: fix build failure on x86/Voyager

Before:

| commit 329513a3
| Author: Yinghai Lu <yhlu.kernel@gmail.com>
| Date:   Wed Jul 2 18:54:40 2008 -0700
|
|     x86: move prefill_possible_map calling early

prefill_possible_mask() was hidden under CONFIG_HOTPLUG_CPU rendering
it invisitble to voyager.  Since this commit it's exposed, but not
provided by the voyager subarch, so add a dummy stub to fix the link
breakage.
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>