[PATCH] fix build on x86_64 with !CONFIG_HOTPLUG_CPU

The commit e2c03888 added
setup_additional_cpus to setup.c but this is only defined if
CONFIG_HOTPLUG_CPU is set.  This patch changes the #ifdef to reflect that.

Signed-off-by: Brian Magnuson <magnuson@rcn.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>