Bump third_party/pigweed/repo from c12c17d to 8ffdd15

Bumps [third_party/pigweed/repo](https://github.com/google/pigweed) from `c12c17d` to `8ffdd15`.
- [Commits](google/pigweed@c12c17d...8ffdd15)

---
updated-dependencies:
- dependency-name: third_party/pigweed/repo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

third_party/pigweed/repo

@@ -0,0 +1 @@
+8ffdd1514e11f6a6ba24b8620a12f922c28f6888