x86, acpi: cleanup, temp_stack is used only when CONFIG_SMP is set

fix:

  arch/x86/kernel/acpi/sleep.c:24: warning: 'temp_stack' defined but not used

[ Sven Wegener <sven.wegener@stealer.net>: fix build bug ]

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>