Bump external/libbtf from 21f05ef to 3c0cbd6

Bumps [external/libbtf](https://github.com/Alan-Jowett/libbtf) from `21f05ef` to `3c0cbd6`.
- [Release notes](https://github.com/Alan-Jowett/libbtf/releases)
- [Commits](Alan-Jowett/libbtf@21f05ef...3c0cbd6)

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

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