QDockWidgetLayout: mark the ctor explicit

A QWidget* is not a faithful representation of a QDockWidgetLayout, so
the ctor should be explicit, even if this is but a private class.

Amends the start of the public history.

Pick-to: 6.8 6.5 5.15
Change-Id: Ib69d72540f094542dbcfc0684e2eec5f90aa532a
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit 21c9aac20abd700cf9f4bcacbbbe7751eb0aa364)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qdockwidget_p.h

@@ -123,7 +123,7 @@ class Q_WIDGETS_EXPORT QDockWidgetLayout : public QLayout
 {
     Q_OBJECT
 public:
-    QDockWidgetLayout(QWidget *parent = nullptr);
+    explicit QDockWidgetLayout(QWidget *parent = nullptr);
     ~QDockWidgetLayout();
     void addItem(QLayoutItem *item) override;
     QLayoutItem *itemAt(int index) const override;