Add Q_FALLTHROUGH to QtDBus tests

There was only one place that needed it.

Change-Id: I067fcfe299b34ab6a771fffd14bf2945f1953d10
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

tests/auto/dbus/qdbusabstractadaptor/myobject.h

@@ -242,10 +242,13 @@ public:
         {
         case 4:
             if4 = new Interface4(this);
+            Q_FALLTHROUGH();
         case 3:
             if3 = new Interface3(this);
+            Q_FALLTHROUGH();
         case 2:
             if2 = new Interface2(this);
+            Q_FALLTHROUGH();
         case 1:
             if1 = new Interface1(this);
         }
@@ -270,4 +273,4 @@ signals:
     void nonScriptableSignalVoid();
 };
 
-#endif // MYOBJECT_H
+#endif // MYOBJECT_H