game_list: Make ContainsAllWords an internally linked non-member function

This function actually depends on no internal class state, so it doesn't
even need to be a part of the class interface.

src/yuzu/game_list.cpp

@@ -138,7 +138,7 @@ GameList::SearchField::SearchField(GameList* parent) : QWidget{parent} {
  * @param userinput String containing all words getting checked
  * @return true if the haystack contains all words of userinput
  */
-bool GameList::ContainsAllWords(const QString& haystack, const QString& userinput) const {
+static bool ContainsAllWords(const QString& haystack, const QString& userinput) {
     const QStringList userinput_split =
         userinput.split(' ', QString::SplitBehavior::SkipEmptyParts);
 

src/yuzu/game_list.h

@@ -89,7 +89,6 @@ private:
 
     void PopupContextMenu(const QPoint& menu_location);
     void RefreshGameDirectory();
-    bool ContainsAllWords(const QString& haystack, const QString& userinput) const;
 
     SearchField* search_field;
     GMainWindow* main_window = nullptr;