The libbpf_util.h is used by xsk.h, so add it to
the install headers.
Reported-by: NBen Pfaff <blp@ovn.org>
Signed-off-by: NWilliam Tu <u9012063@gmail.com>
Acked-by: NYonghong Song <yhs@fb.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>