[IA64] Fix UP build with BSP removal support.

Causes undefined force_cpei_retarget defined in arch/ia64/kernel/smpboot.c
Push the unneeded code inside #ifdef CONFIG_HOTPLUG_CPU.

Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>