Bump github.com/secure-systems-lab/go-securesystemslib (#516)

Bumps [github.com/secure-systems-lab/go-securesystemslib](https://github.com/secure-systems-lab/go-securesystemslib) from 0.1.0 to 0.2.0.
- [Release notes](https://github.com/secure-systems-lab/go-securesystemslib/releases)
- [Commits](https://github.com/secure-systems-lab/go-securesystemslib/compare/v0.1.0...v0.2.0

)

---
updated-dependencies:
- dependency-name: github.com/secure-systems-lab/go-securesystemslib
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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