Bad span for public foreign function

The pub keyword isn't included in the span.

See [this example in the rust playground](https://play.rust-lang.org/?code=extern%20{%0A%20%20pub%20fn%20foxxxo%28%29%3B%0A%20%20pub%20fn%20foxxxo%28%29%3B%0A}%0A%0Apub%20fn%20foyyyo%28%29%20{}%0Apub%20fn%20foyyyo%28%29%20{}%0A%0Afn%20main%28%29%20{%0A%20%20%20%20println!%28%22hello%2C%20world!%22%29%3B%0A}%0A&version=nightly).
