ACPI / AC: fix AC driver compile error when CONFIG_PM_SLEEP is undefined

The ACPI AC driver defines acpi_ac_resume() when CONFIG_PM_SLEEP is
defined.  This results in the following compile error when
CONFIG_PM_SLEEP is undefined:

drivers/acpi/ac.c:248:8: error: ‘acpi_ac_resume’ undeclared here (not in a function)

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>