Fix hidden attribute

[steveklabnik]

Reported on the rust discord:

I am not 100% sure this works but I think so?

[jyn514]

Test failure looks unrelated.

failures:

---- ord::map::test::remove_alien stdout ----
##[error]thread 'ord::map::test::remove_alien' panicked at 'Chunk::push_back: can't push to full chunk', /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/macros.rs:13:23
error: test failed, to rerun pass '--lib'

[jplatte]

Just wanted to submit the same PR. I guess this is no longer maintained? 😕