Add an example to the FFI guide

This is a little short, but the FFI guide needs a cohesive go-over, and this at least adds the information in the meantime.

It also modernizes the style in a small way, by moving the tilde syntax to the grave syntax.
@aturon says no official one, but:

```bash
steve@warmachine:~/src/rust$ git grep 'extern {' | wc -l
253
steve@warmachine:~/src/rust$ git grep 'extern "C" {' | wc -l
25
```

So, seems legit. :)