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: NAnders Roxell <anders.roxell@linaro.org>
Acked-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/config

@@ -19,3 +19,4 @@ CONFIG_CRYPTO_SHA256=m
 CONFIG_VXLAN=y
 CONFIG_GENEVE=y
 CONFIG_NET_CLS_FLOWER=m
+CONFIG_LWTUNNEL=y