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

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

drivers/acpi/battery.c:847:8: error: ‘acpi_battery_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/battery.c

@@ -841,6 +841,8 @@ static int acpi_battery_resume(struct device *dev)
 	acpi_battery_update(battery);
 	return 0;
 }
+#else
+#define acpi_battery_resume NULL
 #endif
 
 static SIMPLE_DEV_PM_OPS(acpi_battery_pm, NULL, acpi_battery_resume);