savegame ex.: make Level ctor explicit

A QString is not a full representation of a Level, so the
Level(QString) ctor should be explicit.

Task-number: QTBUG-108857
Change-Id: I24b705139e61c4aaf59cb0aad3b536013e0d07df
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit fa55d46e1f09ddf45dbe5700d4637b003363c559)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/corelib/serialization/savegame/level.h

@@ -14,7 +14,7 @@ class Level
 {
 public:
     Level() = default;
-    Level(const QString &name);
+    explicit Level(const QString &name);
 
     QString name() const;