Make QGenericRunnable a private nested class of QRunnable

We don't intend to make QGenericRunnable public API, so prevent people
from using it outside QRunnable::create().

Change-Id: I49f182463ca307aaf6370adfa43bae14b4ce0df8
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qrunnable.h

@@ -37,9 +37,10 @@ public:
 
 private:
     static Q_DECL_COLD_FUNCTION QRunnable *warnNullCallable();
+    class QGenericRunnable;
 };
 
-class QGenericRunnable : public QRunnable
+class QRunnable::QGenericRunnable : public QRunnable
 {
     // Type erasure, to only instantiate a non-virtual class per Callable:
     class HelperBase