allow(dead_code) does not work for static externs

The following Rust code:

```rust
extern "C" {
    #[allow(dead_code)]
    static Qt: u64;
}

fn main() {
    println!("hello world");
}
```

produces the warning:

```
warning: foreign static item is never used: `Qt`, #[warn(dead_code)] on by default
```
in spite of the `allow(dead_code)` annotation.