Add a comment stating QMutex::isRecursive should be made const in Qt6

Change-Id: I452b0764790112c59af77bc8d95f403ff37cbc4a
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

src/corelib/thread/qmutex.h

@@ -77,7 +77,7 @@ public:
         return fastTryLock();
     }
 
-    bool isRecursive();
+    bool isRecursive(); //### Qt6: mark const
 
 private:
     inline bool fastTryLock() Q_DECL_NOTHROW {