tokio: fix warnings in AS_FD_PROBE

[djc]

Motivation

The AsFd implementations did not actually appear when building with rustc 1.63. The previous state of the probe generated a warning which resulted in tokio_no_as_fd being generated:

[tokio 1.28.0] error[E0658]: attributes on expressions are experimental
[tokio 1.28.0]  --> <anon>:3:5
[tokio 1.28.0]   |
[tokio 1.28.0] 3 |     #![allow(unused_imports)]
[tokio 1.28.0]   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
[tokio 1.28.0]   |
[tokio 1.28.0]   = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
[tokio 1.28.0] 
[tokio 1.28.0] error: aborting due to previous error
[tokio 1.28.0] 
[tokio 1.28.0] For more information about this error, try `rustc --explain E0658`.
[tokio 1.28.0] cargo:rustc-cfg=tokio_no_as_fd

Solution

Instead, add the allow to each import separately.

Tested this locally with rust 1.63 and it seemed to have the desired effect.

[taiki-e]

Thanks!