qt4: fix memleak.

(cherry picked from commit 606280428d7e03a30779ee21ee4e088c27a34d1b)
Signed-off-by: Rémi Duraffort <ivoire@videolan.org>

modules/gui/qt4/dialogs/plugins.cpp

@@ -142,6 +142,7 @@ inline void PluginTab::FillTree()
         QTreeWidgetItem *item = new PluginTreeItem( qs_item );
         treePlugins->addTopLevelItem( item );
     }
+    module_list_free( p_list );
 }
 
 void PluginTab::search( const QString& qs )