test_rust fails on osx with "can't find crate for `libc`"

After I apply mix fix for #24642 (moved), I see this message in test-suite.log from test_rust.sh:

   Doc-tests protover
error[E0463]: can't find crate for `libc`
  --> /Users/nickm/src/tor/src/rust/protover/lib.rs:25:1
   |
25 | extern crate libc;
   | ^^^^^^^^^^^^^^^^^^ can't find crate

I do not see the same message on my linux rust builds.