[IA64] fix iosapic_remove build error for !HOTPLUG

This patch removes the following stupid compile error that happens
when CONFIG_HOTPLUG is not defined on ia64.

     arch/ia64/kernel/built-in.o(.text+0x712): In function `acpi_unregister_ioapic':
     : undefined reference to `iosapic_remove'

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>