Qt: don't show Ml button if it does nothing

(cherry picked from commit 8eb8d941)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/simple_preferences.cpp

@@ -539,6 +539,8 @@ SPrefsPanel::SPrefsPanel( intf_thread_t *_p_intf, QWidget *_parent,
 #endif
 #ifdef MEDIA_LIBRARY
             BUTTONACT( ui.sqlMLbtn, configML() );
+#else
+            ui.sqlMLbtn->hide();
 #endif
 
             /* interface */