Work around new dead_code warning

[dtolnay]

Warning is new in nightly-2024-01-06 due to rust-lang/rust#118297.

warning: field `0` is never read
  --> src/marker.rs:15:39
   |
15 | pub(crate) struct ProcMacroAutoTraits(Rc<()>);
   |                   ------------------- ^^^^^^
   |                   |
   |                   field in this struct
   |
   = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
15 | pub(crate) struct ProcMacroAutoTraits(());
   |                                       ~~