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: 0e28cbb895
Link: https://lore.kernel.org/all/20250414195928.129040-4-benno.lossin@proton.me
Signed-off-by: Benno Lossin <benno.lossin@proton.me>

rust/pin-init/examples/pthread_mutex.rs

@@ -42,7 +42,7 @@ mod pthread_mtx {
 
     #[derive(Debug)]
     pub enum Error {
-        #[expect(dead_code)]
+        #[allow(dead_code)]
         IO(std::io::Error),
         Alloc,
     }