make pub_use_of_private_extern_crate show up in cargo's future breakage reports
This has been a lint for many years.
However, turns out that outright removing it right now would lead to [tons of crater regressions](https://github.com/rust-lang/rust/pull/127656#issuecomment-2233288534) due to crates depending on an ancient version of `bitflags`. So for now this PR just makes this future-compat lint show up in cargo's reports, so people are warned when they use a dependency that is affected by this.
r? `@petrochenkov`