Bump vendor/vmlinux.h in the submodules group

Bumps the submodules group with 1 update: [vendor/vmlinux.h](https://github.com/libbpf/vmlinux.h).


Updates `vendor/vmlinux.h` from `8f91e9f` to `991dd4b`
- [Commits](libbpf/vmlinux.h@8f91e9f...991dd4b)

---
updated-dependencies:
- dependency-name: vendor/vmlinux.h
  dependency-version: 991dd4b8dfd8c9d62ce8999521b24f61d9b7fc52
  dependency-type: direct:production
  dependency-group: submodules
...

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

Author: dependabot[bot]