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

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

drivers/acpi/button.c:85:8: error: ‘acpi_button_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/button.c

@@ -80,6 +80,8 @@ static void acpi_button_notify(struct acpi_device *device, u32 event);
 
 #ifdef CONFIG_PM_SLEEP
 static int acpi_button_resume(struct device *dev);
+#else
+#define acpi_button_resume NULL
 #endif
 static SIMPLE_DEV_PM_OPS(acpi_button_pm, NULL, acpi_button_resume);