Bump external/SPIRV-Headers from 7f1d2f4 to 8e2ad27 (#66)

Bumps [external/SPIRV-Headers](https://github.com/KhronosGroup/SPIRV-Headers) from `7f1d2f4` to `8e2ad27`.
- [Commits](KhronosGroup/SPIRV-Headers@7f1d2f4...8e2ad27)

---
updated-dependencies:
- dependency-name: external/SPIRV-Headers
  dependency-type: direct:production
...

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