game_list: Make the AddEntry argument a const reference

appendRow takes a QList by const reference, so it doesn't need to be
modifiable.

src/citra_qt/game_list.h

@@ -34,7 +34,7 @@ public:
     void LoadInterfaceLayout();
 
 public slots:
-    void AddEntry(QList<QStandardItem*> entry_items);
+    void AddEntry(const QList<QStandardItem*>& entry_items);
 
 private slots:
     void ValidateEntry(const QModelIndex& item);