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>

drivers/acpi/ac.c

@@ -243,6 +243,8 @@ static int acpi_ac_resume(struct device *dev)
 		kobject_uevent(&ac->charger.dev->kobj, KOBJ_CHANGE);
 	return 0;
 }
+#else
+#define acpi_ac_resume NULL
 #endif
 static SIMPLE_DEV_PM_OPS(acpi_ac_pm_ops, NULL, acpi_ac_resume);