Include our own version of linux/bpf.h to avoid compile errors

Otherwise, the <linux/bpf.h> include file will get picked up from the
system version, which is most likely older. This results in compile errors
with undefined values from the function definitions bpf_helpers.h. If we
carry our own version, we can make sure the two are always in sync.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: tohojo