Bump afl from 0.14.5 to 0.15.0 in /fuzz/afl.rs

Bumps [afl](https://github.com/rust-fuzz/afl.rs) from 0.14.5 to 0.15.0.
- [Release notes](https://github.com/rust-fuzz/afl.rs/releases)
- [Changelog](https://github.com/rust-fuzz/afl.rs/blob/master/CHANGES.md)
- [Commits](https://github.com/rust-fuzz/afl.rs/commits)

---
updated-dependencies:
- dependency-name: afl
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>