QDialog: Add Q_ENUM to DialogCode

This helps the dynamic Python bindings project.

Change-Id: Ibe2c06f4a4e5fd9c3ca41884d811990ec9328b80
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/widgets/dialogs/qdialog.h

@@ -28,6 +28,7 @@ public:
     ~QDialog();
 
     enum DialogCode { Rejected, Accepted };
+    Q_ENUM(DialogCode)
 
     int result() const;