kernel: silence `-Wextra-semi`

silencing gcc's `-Wextra-semi`

Pick-to: 6.5 6.6 6.7
Change-Id: I0e70465585be5ba4d90c691cd0fbd43e1288e694
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty_p.h

@@ -166,7 +166,7 @@ struct QPropertyObserverPointer
     {
         Q_ASSERT(ptr->next.tag() == QPropertyObserver::ObserverNotifiesBinding);
         return ptr->binding;
-    };
+    }
 
 private:
     void unlink_common()