Bump markup5ever from 0.12.0 to 0.12.1 in /native/html5ever_nif (#146)

Bumps [markup5ever](https://github.com/servo/html5ever) from 0.12.0 to 0.12.1.
- [Commits](servo/html5ever@markup5ever-v0.12.0...markup5ever-v0.12.1)

---
updated-dependencies:
- dependency-name: markup5ever
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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