selftests: bpf: add config fragment LWTUNNEL

When test_lwt_seg6local.sh was added commit c99a84ea


("selftests/bpf: test for seg6local End.BPF action") config fragment
wasn't added, and without CONFIG_LWTUNNEL enabled we see this:
Error: CONFIG_LWTUNNEL is not enabled in this kernel.
selftests: test_lwt_seg6local [FAILED]

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>