ACPI: fix unused function warning

CONFIG_ACPI_PROCFS=n:

drivers/acpi/processor_idle.c:83: warning: 'us_to_pm_timer_ticks' defined but not used.
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLen Brown <len.brown@intel.com>

drivers/acpi/processor_idle.c

@@ -80,10 +80,13 @@ module_param(nocst, uint, 0000);
 static unsigned int latency_factor __read_mostly = 2;
 module_param(latency_factor, uint, 0644);
 
+#ifdef CONFIG_ACPI_PROCFS
 static u64 us_to_pm_timer_ticks(s64 t)
 {
 	return div64_u64(t * PM_TIMER_FREQUENCY, 1000000);
 }
+#endif
+
 /*
  * IBM ThinkPad R40e crashes mysteriously when going into C2 or C3.
  * For now disable this. Probably a bug somewhere else.