benchmark: Correct constructor for freelist

Updates to use current constructor for freelist, which was changed
under pr #12644

Ref: https://github.com/nodejs/node/pull/12644
PR-URL: https://github.com/nodejs/node/pull/14627
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

benchmark/misc/freelist.js

@@ -9,7 +9,7 @@ var bench = common.createBenchmark(main, {
 });
 
 function main(conf) {
-  const FreeList = require('internal/freelist').FreeList;
+  const FreeList = require('internal/freelist');
   var n = conf.n;
   var poolSize = 1000;
   var list = new FreeList('test', poolSize, Object);