QFutureInterface<void>: const-correct reportFinished()

The primary QFutureInterface::reportFinished() takes a const T *, so should the
void sepecialisation.

Change-Id: I4d3173f4fd4f8d27baecd407369659a42445aed7
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/concurrent/qfutureinterface.h

@@ -302,7 +302,7 @@ public:
 
     void reportResult(const void *, int) { }
     void reportResults(const QVector<void> &, int) { }
-    void reportFinished(void * = 0) { QFutureInterfaceBase::reportFinished(); }
+    void reportFinished(const void * = 0) { QFutureInterfaceBase::reportFinished(); }
 };
 
 QT_END_NAMESPACE