Backport ATLASG-1450 fix to 21.0

Make prescale tool selections deterministic

PhysicsAnalysis/DerivationFramework/DerivationFrameworkTools/DerivationFrameworkTools/PrescaleTool.h

@@ -25,7 +25,7 @@ namespace DerivationFramework {
       virtual bool eventPassesFilter() const;
 
     private:
-      mutable int m_prescale, m_eventCounter;	
+      unsigned int m_prescale;	
   }; 
 }