QGtk3Interface: code tidies

The class is not meant to be copied. Therefore disable copy/moves.
This gets rid of a warning in QMetaType, since the class is also a
Q_GADGET.

Change-Id: Ie463ebd0c0dac9f2c37c4ffa88e1920ad313fd00
Reviewed-by: Thorbjørn Lindeijer <bjorn@lindeijer.nl>

src/plugins/platformthemes/gtk3/qgtk3interface_p.h

@@ -53,6 +53,7 @@ class QGtk3Storage;
 class QGtk3Interface
 {
     Q_GADGET
+    Q_DISABLE_COPY_MOVE(QGtk3Interface)
 public:
     QGtk3Interface(QGtk3Storage *);
     ~QGtk3Interface();