QQuickRhiItem: make (Private&, parent) ctor explicit

Found in API-review.

Amends 8e6d3d8.

Pick-to: 6.9
Change-Id: Ia0dd506eea3fd469050ded143acbb3bc824ec05f
Reviewed-by: Kaj Grönholm <kaj.gronholm@qt.io>

src/quick/items/qquickrhiitem.h

@@ -103,7 +103,7 @@ class Q_QUICK_EXPORT QQuickRhiItem : public QQuickItem
     void effectiveColorBufferSizeChanged();
 
 protected:
-    QQuickRhiItem(QQuickRhiItemPrivate &dd, QQuickItem *parent = nullptr);
+    explicit QQuickRhiItem(QQuickRhiItemPrivate &dd, QQuickItem *parent = nullptr);
 
     virtual QQuickRhiItemRenderer *createRenderer() = 0;