selftests net: fix bpf build error

selftests net: fix bpf build error

bpf_helpers.h has been moved to tools/lib/bpf since 5.10, so add more
including path.

Fixes: 68311159154a ("selftests net: add UDP GRO fraglist + bpf self-tests")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Lina Wang <lina.wang@mediatek.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20220606064517.8175-1-lina.wang@mediatek.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>