benches: fix build error

[taiki-e]

$ cargo build --benches --manifest-path benches/Cargo.toml
   Compiling benches v0.0.0 (...)
error[E0432]: unresolved import `tokio::stream::StreamExt`
 --> benches/fs.rs:3:5
  |
3 | use tokio::stream::StreamExt;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^ no `StreamExt` in `stream`

error[E0599]: no method named `next` found for struct `FramedRead<tokio::fs::File, BytesCodec>` in the current scope
  --> benches/fs.rs:35:43
   |
35 |                 let _bytes = input_stream.next().await.unwrap();
   |                                           ^^^^ method not found in `FramedRead<tokio::fs::File, BytesCodec>`

error: aborting due to 2 previous errors