Add missing selector.wakeup() in SmackRactor.schedule()

Without this, newly scheduled runnables would potentially not be
scheduled in case the reactor thread was blocking in select(0)
indefinetly.

Thanks to Eng ChongMeng for reporting this.

smack-core/src/main/java/org/jivesoftware/smack/SmackReactor.java

@@ -150,6 +150,7 @@ public class SmackReactor {
         Date releaseTimeDate = new Date(releaseTimeEpoch);
         ScheduledAction scheduledAction = new ScheduledAction(runnable, releaseTimeDate, this);
         scheduledActions.add(scheduledAction);
+        selector.wakeup();
         return scheduledAction;
     }