AchievementManager: Return by reference from GetLock()

This makes the API a little nicer to use, since you don't need to do
a mandatory dereference when passing the lock into any kind of scope
guard.

Source/Core/DolphinQt/Achievements/AchievementHeaderWidget.cpp

@@ -62,7 +62,7 @@ AchievementHeaderWidget::AchievementHeaderWidget(QWidget* parent) : QWidget(pare
   m_total->setAlignment(Qt::AlignTop);
   setLayout(m_total);
 
-  std::lock_guard lg{*AchievementManager::GetInstance().GetLock()};
+  std::lock_guard lg{AchievementManager::GetInstance().GetLock()};
   UpdateData();
 }