PM / devfreq: remove compiler error when a governor is module

With the intruction of patch, eff607fdb1f787da1fedf46ab6e64adc2afd1c5a,
it became possible to include a governor as a module.
Thus the #ifdef statement for a governor should become #if IS_ENABLED.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

include/linux/devfreq.h

@@ -192,7 +192,7 @@ extern int devfreq_register_opp_notifier(struct device *dev,
 extern int devfreq_unregister_opp_notifier(struct device *dev,
 					   struct devfreq *devfreq);
 
-#ifdef CONFIG_DEVFREQ_GOV_SIMPLE_ONDEMAND
+#if IS_ENABLED(CONFIG_DEVFREQ_GOV_SIMPLE_ONDEMAND)
 /**
  * struct devfreq_simple_ondemand_data - void *data fed to struct devfreq
  *	and devfreq_add_device