Eliminate a warning

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2402 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/drivers/rtc/alarm.c

@@ -74,7 +74,7 @@ static rt_err_t alarm_set(struct rt_alarm *alarm)
     return (ret);
 }
 
-static alarm_wakeup(struct rt_alarm *alarm, struct tm *now)
+static void alarm_wakeup(struct rt_alarm *alarm, struct tm *now)
 {
     rt_uint32_t sec_alarm, sec_now;
     rt_bool_t wakeup = RT_FALSE;