fix: remove kernel source headers from BPF compile step in CI
linux-headers-generic pulls in /usr/src/linux-headers-*-generic/include
which is intended for kernel module compilation. Including it in a
-target bpf build causes clang to follow linux/bpf.h → linux/compiler.h
→ asm/rwonce.h, a header that only exists for host architectures, not
the BPF target — crashing clang with exit code 139.
BPF programs should use the UAPI headers from linux-libc-dev
(/usr/include/linux/) and BPF helpers from libbpf-dev (/usr/include/bpf/).
These are already present on the GitHub Actions runner without any extra
packages. The linux-headers-generic install is removed entirely.