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: NMarcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: NPavel Machek <pavel@suse.cz>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/acpi/sleep.c

@@ -20,7 +20,7 @@ unsigned long acpi_realmode_flags;
 /* address in low memory of the wakeup routine. */
 static unsigned long acpi_realmode;
 
-#ifdef CONFIG_64BIT
+#if defined(CONFIG_SMP) && defined(CONFIG_64BIT)
 static char temp_stack[10240];
 #endif