fixup for v6 by adding missing require()

steal a line from nodejs#7207 to make things work

PR-URL: nodejs#8437
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

Author: addaleax

lib/buffer.js

@@ -4,6 +4,7 @@
 const binding = process.binding('buffer');
 const { isArrayBuffer } = process.binding('util');
 const bindingObj = {};
+const internalUtil = require('internal/util');
 
 class FastBuffer extends Uint8Array {}