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/AchievementProgressWidget.cpp

@@ -29,7 +29,7 @@ AchievementProgressWidget::AchievementProgressWidget(QWidget* parent) : QWidget(
   m_common_layout = new QVBoxLayout();
 
   {
-    std::lock_guard lg{*AchievementManager::GetInstance().GetLock()};
+    std::lock_guard lg{AchievementManager::GetInstance().GetLock()};
     UpdateData();
   }