Don't suggest using fields in a static method

``` rust
fn f(_: bool) {}

struct Foo {
    cx: bool,
}

impl Foo {
    fn bar() {
        f(cx);
    }
}

fn main() {}
```

```
<anon>:9:11: 9:13 error: unresolved name `cx`. Did you mean `self.cx`? [E0425]
<anon>:9         f(cx);
                   ^~
```
