TreeModel: Make TreeItem::m_children private

Change-Id: Ica8ee995fb265d1108fced1e1c1a4c9234783c2b
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/treemodel.h

@@ -136,8 +136,6 @@ private:
 
     TreeItem *m_parent; // Not owned.
     BaseTreeModel *m_model; // Not owned.
-
- protected:
     QVector<TreeItem *> m_children; // Owned.
     friend class BaseTreeModel;
 };