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/VideoCommon/OnScreenUI.cpp

@@ -330,7 +330,7 @@ void OnScreenUI::DrawDebugText()
 #ifdef USE_RETRO_ACHIEVEMENTS
 void OnScreenUI::DrawChallenges()
 {
-  std::lock_guard lg{*AchievementManager::GetInstance().GetLock()};
+  std::lock_guard lg{AchievementManager::GetInstance().GetLock()};
   const auto& challenge_icons = AchievementManager::GetInstance().GetChallengeIcons();
   if (challenge_icons.empty())
     return;