Fix Vala compilation problem
Fix Vala compilation problem

Fixes the following error:
xadapter.vala:42.29-42.80: error: duplicating Display instance, use unowned variable or explicitly invoke copy method
            this.xdisplay = Gdk.X11Display.get_xdisplay (rootwin.get_display ());
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

https://bugzilla.gnome.org/show_bug.cgi?id=655980

GNOME/caribou,GNOME/caribou,GNOME/caribou