clippy: fix unused_trait_names lint

[xtqqczze]

https://rust-lang.github.io/rust-clippy/master/index.html#unused_trait_names

[xtqqczze]

false positive

error[E0405]: cannot find trait `AsFd` in this scope
  --> src/uu/wc/src/count_fast.rs:47:39
   |
47 | fn count_bytes_using_splice(fd: &impl AsFd) -> Result<usize, usize> {
   |                                       ^^^^ not found in this scope
   |
help: consider importing this trait
   |
 7 + use std::os::fd::AsFd;
   |

cc: @profetia

[sylvestre]

it needs to be rebased