Bump tests/external/kissfft from 8f47a67 to f5f2a3b (#3341)

Bumps [tests/external/kissfft](https://github.com/mborgerding/kissfft) from `8f47a67` to `f5f2a3b`.
- [Release notes](https://github.com/mborgerding/kissfft/releases)
- [Commits](mborgerding/kissfft@8f47a67...f5f2a3b)

---
updated-dependencies:
- dependency-name: tests/external/kissfft
  dependency-type: direct:production
...

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