Fix QWaylandDrag leak in the compositor

Make it a scoped pointer.

Change-Id: Iaeef87cad2022496ebe3a7c746b02584fac9a485
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/compositor/wayland_wrapper/qwlinputdevice.cpp

@@ -352,7 +352,7 @@ QWaylandInputDevice *InputDevice::handle() const
 
 QWaylandDrag *InputDevice::dragHandle() const
 {
-    return m_dragHandle;
+    return m_dragHandle.data();
 }
 
 const DataDevice *InputDevice::dataDevice() const

src/compositor/wayland_wrapper/qwlinputdevice_p.h

@@ -128,7 +128,7 @@ public:
 
 private:
     QWaylandInputDevice *m_handle;
-    QWaylandDrag *m_dragHandle;
+    QScopedPointer<QWaylandDrag> m_dragHandle;
     Compositor *m_compositor;
     QWaylandInputDevice::CapabilityFlags m_capabilities;