Qt: preferences_widgets: fix vtable entries for show()/hide()

(cherry picked from commit d73cc541acbf2ba3067ea76aaedf61b7742a01a0)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/preferences_widgets.hpp

@@ -139,8 +139,8 @@ public:
     IntegerConfigControl( vlc_object_t *, module_config_t *,
                           QLabel*, QSlider* );
     virtual int getValue() const;
-    virtual void show() const { spin->show(); if( label ) label->show(); }
-    virtual void hide() const { spin->hide(); if( label ) label->hide(); }
+    virtual void show() { spin->show(); if( label ) label->show(); }
+    virtual void hide() { spin->hide(); if( label ) label->hide(); }
 
 protected:
     QSpinBox *spin;