game_list: Pass the parent constructor argument to the QWidget base class

If the control was ever passed an explicit parent, a potential memory leak
would happen, as the game list wouldn't be freed.

However, in our case, the game list was placed within a layout, which
automatically performs reparenting, avoiding this issue.

src/citra_qt/game_list.cpp

@@ -13,7 +13,7 @@
 #include "game_list_p.h"
 #include "ui_settings.h"
 
-GameList::GameList(QWidget* parent) {
+GameList::GameList(QWidget* parent) : QWidget{parent} {
     QVBoxLayout* layout = new QVBoxLayout;
 
     tree_view = new QTreeView;