Declare QQuickImplicitSizeItem as QML_ANONYMOUS

Holes in the QML type hierarchy lead to interesting effects. We don't
need to give ourselves and our users those headaches.

Pick-to: 6.2
Change-Id: Ie1cf94001fda0e1552a2cf260ccc327c30bf7b31
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/quick/items/qquickimplicitsizeitem_p.h

@@ -62,6 +62,8 @@ class Q_QUICK_PRIVATE_EXPORT QQuickImplicitSizeItem : public QQuickItem
     Q_OBJECT
     Q_PROPERTY(qreal implicitWidth READ implicitWidth NOTIFY implicitWidthChanged)
     Q_PROPERTY(qreal implicitHeight READ implicitHeight NOTIFY implicitHeightChanged)
+    QML_ANONYMOUS
+    QML_ADDED_IN_VERSION(6, 2)
 
 protected:
     QQuickImplicitSizeItem(QQuickImplicitSizeItemPrivate &dd, QQuickItem *parent);