QRunnable: add Q_DISABLE_COPY

...but only for Qt 6. It's a source-incompatible change,
e.g. in a user hierarchy of clone()able runnables.

Change-Id: I8610308dea46da19bda5c96985d35f31c43484be
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qrunnable.h

@@ -45,7 +45,9 @@ class Q_CORE_EXPORT QRunnable
     friend class QThreadPool;
     friend class QThreadPoolPrivate;
     friend class QThreadPoolThread;
-
+#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
+    Q_DISABLE_COPY(QRunnable)
+#endif
 public:
     virtual void run() = 0;