rust: pin-init: examples: use `allow` instead of `expect`

rust: pin-init: examples: use `allow` instead of `expect`

Rust 1.78 doesn't emit a `dead_code` error on the annotated element,
resulting in the `unfulfilled_lint_expectations` error. Rust 1.85 does
emit the `dead_code` error, so we still need an `allow`.

Link: https://github.com/Rust-for-Linux/pin-init/pull/33/commits/0e28cbb895bd29f896a59b40e8ed506ea7bef13c
Link: https://lore.kernel.org/all/20250414195928.129040-4-benno.lossin@proton.me
Signed-off-by: Benno Lossin <benno.lossin@proton.me>