add missing require

src/backupformat/rsync.js

@@ -13,6 +13,7 @@ const assert = require('assert'),
     async = require('async'),
     backupTargets = require('../backuptargets.js'),
     BoxError = require('../boxerror.js'),
+    crypto = require('crypto'),
     DataLayout = require('../datalayout.js'),
     { DecryptStream } = require('../hush.js'),
     debug = require('debug')('box:backupformat/rsync'),