Allow unused type in no_type_leakage test

[sunsided]

A contender for the world's smallest PR.

Right now, the code builds with a warning (for me):

warning: field `0` is never read
  --> tests/no_type_leakage.rs:15:16
   |
15 | pub struct Bar(Foo);
   |            --- ^^^
   |            |
   |            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 struct Bar(());
   |             

This PR makes the type

#[allow(unused)]
pub struct Bar(Foo);

To suppress that warning.

[idanarye]

Thanks!