Ignore new dead_code warning in test

Since nightly-2024-03-12.

    warning: struct `Dst` is never constructed
      --> tests/test_derive.rs:63:8
       |
    63 | struct Dst {
       |        ^^^
       |
       = note: `#[warn(dead_code)]` on by default

Author: dtolnay

tests/test_derive.rs

@@ -60,6 +60,7 @@ struct Test<'a> {
 }
 
 #[derive(Serialize)]
+#[allow(dead_code)]
 struct Dst {
     #[serde(with = "serde_bytes")]
     bytes: [u8],