Avoid error claiming Add(usize) is dead code

Clippy v0.1.78 identifies this as dead code. However, further down in
the same file, there is clearly a user:

  impl Handler<Result, Result> for Add {

This might be yet another incarnation of
rust-lang/rust#56750

Let's just mark it as intentionally dead-code, even if this is untrue,
to make clippy happy again.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

lychee-lib/src/chain/mod.rs

@@ -188,6 +188,7 @@ mod test {
     };
     use async_trait::async_trait;
 
+    #[allow(dead_code)] // work-around
     #[derive(Debug)]
     struct Add(usize);