DolphinQt2/NetPlay/GameListDialog: Make GetSelectedUniqueID() const qualified

Given this member function returns a const reference to a QString, it
can be made const qualified, since it doesn't modify internal state.

Source/Core/DolphinQt2/NetPlay/GameListDialog.cpp

@@ -59,7 +59,7 @@ void GameListDialog::PopulateGameList()
   m_game_list->sortItems();
 }
 
-const QString& GameListDialog::GetSelectedUniqueID()
+const QString& GameListDialog::GetSelectedUniqueID() const
 {
   return m_game_id;
 }

Source/Core/DolphinQt2/NetPlay/GameListDialog.h

@@ -18,7 +18,7 @@ public:
   explicit GameListDialog(QWidget* parent);
 
   int exec() override;
-  const QString& GetSelectedUniqueID();
+  const QString& GetSelectedUniqueID() const;
 
 private:
   void CreateWidgets();