selftests/bpf: Use vmlinux.h for BPF programs

Some of the bpf test progs still use linux/libc headers.
Let's use vmlinux.h instead like the rest of test progs.
This will also ease cross compiling.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250821030254.398826-1-hengqi.chen@gmail.com