Fix incremental builds of core by allowing unused attribute.
Fix incremental builds of core by allowing unused attribute.

The same problem as in https://github.com/rust-lang/rust/issues/65023 was
introduced by https://github.com/rust-lang/rust/pull/67657. This works
around the current incrcomp issue with these attributes by allowing it
here.

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust