Cleanup warnings of -Wswitch

Add default branch to the switch-case.

Source/Core/DolphinQt/GameList/GameList.cpp

@@ -747,6 +747,7 @@ void GameList::OpenGCSaveFolder()
       break;
     }
     case ExpansionInterface::EXIDEVICE_MEMORYCARD:
+    {
       std::string memcard_path = i == 0 ? Config::Get(Config::MAIN_MEMCARD_A_PATH) :
                                           Config::Get(Config::MAIN_MEMCARD_B_PATH);
 
@@ -756,6 +757,9 @@ void GameList::OpenGCSaveFolder()
       url = QUrl::fromLocalFile(QString::fromStdString(memcard_dir));
       break;
     }
+    default:
+      break;
+    }
 
     found |= !url.isEmpty();