Mainline UML Rust stable io.rs unused imports

[Miguel Ojeda]

Hi all,

In mainline and next, for UML with Rust stable (but not with Rust
minimum), I am seeing:

      RUSTC L rust/kernel.o - due to target missing
    error: unused import: `define_read`
       --> rust/kernel/io.rs:229:16
        |
    229 | pub(crate) use define_read;
        |                ^^^^^^^^^^^
        |
        = note: `-D unused-imports` implied by `-D warnings`
        = help: to override `-D warnings` add `#[allow(unused_imports)]`

    error: unused import: `define_write`
       --> rust/kernel/io.rs:262:16
        |
    262 | pub(crate) use define_write;
        |                ^^^^^^^^^^^

Cheers,
Miguel