futures-util: Add derive(Debug) to WakerToHandle

Fixes this error on ToT:
```
error: type does not implement `Debug`; consider adding `#[derive(Debug)]` or a manual implementation
   --> futures-util/src/compat/compat01as03.rs:325:1
    |
325 | struct WakerToHandle<'a>(&'a task03::Waker);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D missing-debug-implementations` implied by `-D warnings`
```

Tested:
- `cargo fmt`.
- `cargo doc`.
- `cargo test`.

futures-util/src/compat/compat01as03.rs

@@ -321,7 +321,7 @@ where
 
 struct NotifyWaker(task03::Waker);
 
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 struct WakerToHandle<'a>(&'a task03::Waker);
 
 impl From<WakerToHandle<'_>> for NotifyHandle01 {