fix namespaced build. *again*

Change-Id: I3cbc3efdac61a9f329d0796c64885e6565bd0b77
Reviewed-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>

src/client/global/qwaylandclientextension.h

@@ -40,6 +40,7 @@
 #include <QObject>
 #include <QtWaylandClient/qwaylandclientexport.h>
 
+struct wl_interface;
 struct wl_registry;
 
 QT_BEGIN_NAMESPACE