xdp-tools: don't rely on host bpf headers

xdp-tools: don't rely on host bpf headers

xdp-tools build currently breaks on build hosts which do not have
libbpf headers installed because the build system wrongly tries to
use the host's include path.
Properly pass path to libbpf headers to xdp-tools build system to
fix build e.g. on the buildbots.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>