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: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>