Add missing use statement for MiddlewareRunner after moving to Io subnamespace

Author: jsor

src/Server.php

@@ -3,6 +3,7 @@
 namespace React\Http;
 
 use React\Http\Io\IniUtil;
+use React\Http\Io\MiddlewareRunner;
 use React\Http\Middleware\LimitConcurrentRequestsMiddleware;
 use React\Http\Middleware\RequestBodyBufferMiddleware;
 use React\Http\Middleware\RequestBodyParserMiddleware;