Input: st-keyscan - fix 'defined but not used' compiler warnings

Add #ifdef CONFIG_PM_SLEEP around keyscan_supend() and keyscan_resume() to
fix the following compiler warnings occuring if CONFIG_PM_SLEEP is unset:

+ /scratch/kisskb/src/drivers/input/keyboard/st-keyscan.c: warning: 'keyscan_resume' defined but not used [-Wunused-function]: => 235:12
+ /scratch/kisskb/src/drivers/input/keyboard/st-keyscan.c: warning: 'keyscan_suspend' defined but not used [-Wunused-function]: => 218:12

Reported-by: Geert Uytterhoeven <[e-mail]>
Link: https://lkml.org/lkml/2014/7/8/109
Signed-off-by: Tobias Klauser <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/keyboard/st-keyscan.c

···
 	return 0;
 }
 
+#ifdef CONFIG_PM_SLEEP
 static int keyscan_suspend(struct device *dev)
 {
 	struct platform_device *pdev = to_platform_device(dev);
···
 	mutex_unlock(&input->mutex);
 	return retval;
 }
+#endif
 
 static SIMPLE_DEV_PM_OPS(keyscan_dev_pm_ops, keyscan_suspend, keyscan_resume);