benchmark: use strict mode

Apply strict mode to benchmark code.

PR-URL: https://github.com/nodejs/node/pull/5336
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

benchmark/querystring/querystring-stringify.js

@@ -1,3 +1,4 @@
+'use strict';
 var common = require('../common.js');
 var querystring = require('querystring');
 var v8 = require('v8');