Add a default argument to QDBusPendingCallWatcher::finished signal

So we can do
  connect(&watcher, SIGNAL(finished()), receiver, SLOT(foo()));

Change-Id: I39cc61d0d59846ab8c23ffff14241d33fecf2d53
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/dbus/qdbuspendingcall.h

@@ -106,7 +106,7 @@ public:
     void waitForFinished();     // non-virtual override
 
 Q_SIGNALS:
-    void finished(QDBusPendingCallWatcher *self);
+    void finished(QDBusPendingCallWatcher *self = Q_NULLPTR);
 
 private:
     Q_DECLARE_PRIVATE(QDBusPendingCallWatcher)