QWidget: unbreak QT_NO_OPENGL builds

Amends 94d7603d51.

The port from QVector<QPlatformTextureList*> to a container of
unique_ptr<QPlatformTextureList> uncovered that QPlatformTextureList
isn't defined for QT_NO_OPENGL builds.

Some unguarded forward-declarations made the old declaration compile
by accident. The new code caught this, so add the #ifdef that had been
missing all along.

Change-Id: If3b14fc24007b1c917a41ab83343c2e5e65fc643
Reviewed-by: Martin Storsjö <martin@martin.st>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Tasuku Suzuki <tasuku.suzuki@qbc.io>

src/widgets/kernel/qwidget_p.h

@@ -189,7 +189,9 @@ struct QTLWExtra {
     // ### TODO replace initialScreenIndex with QScreen *, in case the screens change at runtime
     int initialScreenIndex; // Screen number when passing a QDesktop[Screen]Widget as parent.
 
+#ifndef QT_NO_OPENGL
     std::vector<std::unique_ptr<QPlatformTextureList>> widgetTextures;
+#endif
 
     // *************************** Cross-platform bit fields ****************************
     uint opacity : 8;