Temporary change to prepare for qtbase change

QPlatformDrag::platformDropData() will be removed in qtbase. By
removing the 'override' specifier, we make sure that qtwayland will
compile with both versions, so we can get the changes through the
CI system.

Change-Id: I8f5f3252ecc5125e5dab3f7a4da8760fe259b739
Reviewed-by: Albert Astals Cid <albert.astals@canonical.com>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/plugins/platforms/wayland/qwaylanddnd_p.h

@@ -71,7 +71,7 @@ public:
     QWaylandDrag(QWaylandDisplay *display);
     ~QWaylandDrag();
 
-    QMimeData *platformDropData() override;
+    QMimeData *platformDropData();
 
     void updateTarget(const QString &mimeType);
     void setResponse(const QPlatformDragQtResponse &response);