Fix third-party/rust/Cargo.toml

Summary:
It looks like D56546196 was incomplete. D56546196 added a new
dependency, but didn't account for the case where the outside world
tries to build buck2.

This diff fixes that.

Reviewed By: JakobDegen

Differential Revision: D56766826

fbshipit-source-id: 3590056f3c5032bdc81d40855b1bf939a4686659