Ignore clippy::unused_unit lint

```
warning: unneeded unit return type
    --> src/lib.rs:1159:42
     |
1159 |     struct AsyncCallOnDrop<Fut, Cleanup: FnMut()> {
     |                                          ^^^^^^^ help: remove the `-> ()`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit
     = note: `#[warn(clippy::unused_unit)]` on by default
```

Author: taiki-e

src/lib.rs

@@ -38,6 +38,7 @@
     html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
 )]
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
+#![allow(clippy::unused_unit)] // false positive fixed in Rust 1.89
 
 use std::fmt;
 use std::marker::PhantomData;