xcb: rename arg in QXcbBackingStore ctor

... as it is a window, not widget. This matches the name from
cpp file: QXcbBackingStore::QXcbBackingStore(QWindow *window).

Change-Id: Ib9f69cbe80ea645bbce0e4f5ef24a8df8808b823
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/plugins/platforms/xcb/qxcbbackingstore.h

@@ -54,7 +54,7 @@ class QXcbShmImage;
 class QXcbBackingStore : public QXcbObject, public QPlatformBackingStore
 {
 public:
-    QXcbBackingStore(QWindow *widget);
+    QXcbBackingStore(QWindow *window);
     ~QXcbBackingStore();
 
     QPaintDevice *paintDevice() override;