deny.toml: ignore RUSTSEC-2021-0115 for now
deny.toml: ignore RUSTSEC-2021-0115 for now

Upgrading would currenly involve a bump in MSRV, but there's hope
that a 1.1.x version of zeroize_derive might arrive.

Issue #202 tracks.

project-oak/tink-rust,project-oak/tink-rust,project-oak/tink-rust