allow unused_features lint

[BennoLossin]

The unused_features lint was introduced in rust-lang/rust#152164, presumably landing in Rust 1.96.0. We simplified the way we use features in #102, always enabling them when building the crate with certain features enabled. For this reason allow the lint.

I plan to also add a new CI run that enables the lint & checks if we can remove certain features. Will do so at a later point in time.

This PR fixes the CI failure observed in https://github.com/Rust-for-Linux/pin-init/actions/runs/22754182535

[nbdd0121]

We would also need to enable this in general for other kernel crates, I think?

[BennoLossin]

Yes, I already notified Miguel (although in the kernel it only is a warning as opposed to here where it is an error due to our deny(warnings)).

[nbdd0121]

There's CONFIG_WERROR so it's also problematic

[ojeda]

Yeah, e.g. from one of my builds:

error: feature `lint_reasons` is declared but not used
   --> rust/pin-init/src/lib.rs:267:56
    |
267 | #![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
    |                                                        ^^^^^^^^^^^^
    |
    = note: `-D unused-features` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unused_features)]`

I will put a workaround for rust.docs.kernel.org for now.

[BennoLossin]

I tried to set up a CI check, but it didn't work, since the unused_features lint apparently also lints when a feature is stable and used, see rust-lang/rust#153523