Bump external/libbtf from 81e0a6f to 21f05ef

Bumps [external/libbtf](https://github.com/Alan-Jowett/libbtf) from `81e0a6f` to `21f05ef`.
- [Release notes](https://github.com/Alan-Jowett/libbtf/releases)
- [Commits](Alan-Jowett/libbtf@81e0a6f...21f05ef)

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

Signed-off-by: dependabot[bot] <[email protected]>