[PATCH] rust: bug: fix `dead_code` warning in `warn_on!`

[Miguel Ojeda]

On Sat, Aug 8, 2026 at 12:43 AM FUJITA Tomonori <tomo at flapping.org> wrote:
>
> I sent a fix for this bug a few days ago:
>
> https://lore.kernel.org/all/20260801024841.786664-1-tomo@flapping.org/

Ah, right, sorry! I also saw another one with `rusttest` for arm64 and
riscv64. Did you see that one? We probably need a `cfg(testlib)`
split.

Cheers,
Miguel