Bump third-party/verilator/master from d26fcca to a647747 (#860)

Bumps [third-party/verilator/master](https://github.com/verilator/verilator) from `d26fcca` to `a647747`.
- [Commits](verilator/verilator@d26fcca...a647747)

---
updated-dependencies:
- dependency-name: third-party/verilator/master
  dependency-version: a64774726033811dc387c49899e504dba236a5e4
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]