PM / Sleep: Fix build warning in sysfs.c for CONFIG_PM_SLEEP unset

The power/async device sysfs attribute is only used if both
CONFIG_PM_ADVANCED_DEBUG and CONFIG_PM_SLEEP are set, but the code
implementing it doesn't depend on CONFIG_PM_SLEEP.  As a result, a
build warning appears if CONFIG_PM_ADVANCED_DEBUG is set and
CONFIG_PM_SLEEP is not set.

Fix it by adding a #ifdef CONFIG_PM_SLEEP around the code in
question.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/sysfs.c

@@ -474,6 +474,8 @@ static DEVICE_ATTR(runtime_enabled, 0444, rtpm_enabled_show, NULL);
 
 #endif
 
+#ifdef CONFIG_PM_SLEEP
+
 static ssize_t async_show(struct device *dev, struct device_attribute *attr,
 			  char *buf)
 {
@@ -500,6 +502,8 @@ static ssize_t async_store(struct device *dev, struct device_attribute *attr,
 }
 
 static DEVICE_ATTR(async, 0644, async_show, async_store);
+
+#endif
 #endif /* CONFIG_PM_ADVANCED_DEBUG */
 
 static struct attribute *power_attrs[] = {