PerfProfilerFlameGraphModel: Make Payload::m_numSamples mutable

This is necessary to make the class movable.

Change-Id: I85decaa9f5c520aa3272a018ceca3aba642e1565
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/perfprofiler/perfprofilerflamegraphmodel.cpp

@@ -57,7 +57,7 @@ public:
 private:
     const PerfProfilerFlameGraphData *m_parent;
     PerfProfilerFlameGraphModel::Data *m_data;
-    const int m_numSamples;
+    int m_numSamples;
 };
 
 typedef PerfResourceCounter<Payload> ThreadResourceCounter;