submodule: bump openthread from 543e32d to b4c76b3 (#196)

Bumps [openthread](https://github.com/openthread/openthread) from `543e32d` to `b4c76b3`.
- [Commits](openthread/openthread@543e32d...b4c76b3)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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