Bug 1663677 - Fix macro hygiene usage in fxaccounts/rust-bridge. r=eoger,lina

This prevents us from building with rust beta/nightly.

This is technically a Rust regression, see
rust-lang/rust#76480, though I think that this
should've never worked.

Differential Revision: https://phabricator.services.mozilla.com/D89466

Author: emilio