rust: allow `dead_code` for never constructed bindings

Starting with the upcoming Rust 1.80.0 (since upstream commit 35130d7233e9
("Detect pub structs never constructed and unused associated constants
in traits")), the `dead_code` pass detects more cases, which triggers
in the `bindings` crate:

    warning: struct `boot_params` is never constructed
        --> rust/bindings/bindings_generated.rs:10684:12
        |
    10684 | pub struct boot_params {
        |            ^^^^^^^^^^^
        |
        = note: `#[warn(dead_code)]` on by default

As well as in the `uapi` one:

    warning: struct `boot_params` is never constructed
        --> rust/uapi/uapi_generated.rs:10392:12
        |
    10392 | pub struct boot_params {
        |            ^^^^^^^^^^^
        |
        = note: `#[warn(dead_code)]` on by default

These are all expected, since we do not use all the structs in the
bindings that `bindgen` generates from the C headers.

Therefore, allow them.

Reviewed-by: Björn Roy Baron <bjorn3_gh@proto...>