Qt: properly show the dialog buttons on Win32

Close #13648

(cherry picked from commit 339f955e7d0735bc74ed25e14b975e1b7a276855)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/util/qvlcframe.hpp

@@ -152,7 +152,10 @@ class QVLCDialog : public QDialog
 public:
     QVLCDialog( QWidget* parent, intf_thread_t *_p_intf ) :
                                     QDialog( parent ), p_intf( _p_intf )
-    {}
+    {
+        setWindowFlags( Qt::Dialog|Qt::WindowMinMaxButtonsHint|
+                        Qt::WindowSystemMenuHint|Qt::WindowCloseButtonHint );
+    }
     virtual ~QVLCDialog() {};
     void toggleVisible()
     {