racket: add libreadline to ffi libs - enables xrepl.

The Racket derivation already includes a bunch of extra libs in the `ffiSharedLibs` field.  However, `libreadline` was missing.  This adds it.  A test of whether this works is the following:

```
$ racket
Welcome to Racket v6.1.1.
-> (require xrepl)
->
```

If that succeeds without warning or error, you have readline loaded.  Alternatively:

```
$ racket
Welcome to Racket v6.1.1.
-> (require ffi/unsafe)
-> (ffi-lib "libreadline.so")
#<ffi-lib>
```

Note that this patch was built not against head, but against the last working copy of Racket.  Because [Racket 6.1.1 recently started failing its builds](http://hydra.nixos.org/build/16967627).  (cc @samth)