fix: error when preloding is disabled in the browser (#2086)

Sometimes you don't want to preload everything, if your code is an
example or a demo.  In that case you'd turn off preloading but
`setImmediate` is not a thing in browsers-other-than-IE so pull
in `async/set-immediate` instead

src/core/mfs-preload.js

@@ -1,7 +1,7 @@
 'use strict'
 
 const debug = require('debug')
-
+const setImmediate = require('async/setImmediate')
 const log = debug('ipfs:mfs-preload')
 log.error = debug('ipfs:mfs-preload:error')