game_list: Make containsAllWords a const member function

This doesn't actually modify the internal class state, so it can be a
const member function. While we're at it, amend the function to take
its arguments by const reference.

src/yuzu/game_list.h

@@ -89,7 +89,7 @@ private:
 
     void PopupContextMenu(const QPoint& menu_location);
     void RefreshGameDirectory();
-    bool containsAllWords(QString haystack, QString userinput);
+    bool containsAllWords(const QString& haystack, const QString& userinput) const;
 
     SearchField* search_field;
     GMainWindow* main_window = nullptr;