fix: missing readonly

Author: Akos Kitta

arduino-ide-extension/src/browser/boards/boards-service-provider.ts

@@ -143,8 +143,7 @@ export class BoardsServiceProvider
   @inject(ILogger)
   private readonly logger: ILogger;
   @inject(MessageService)
-  private messageService: MessageService;
-
+  private readonly messageService: MessageService;
   @inject(BoardsService)
   private readonly boardsService: BoardsService;
   @inject(CommandService)