selftests/bpf: CONFIG_IPV6_SEG6_BPF required for test_seg6_loop.o

test_seg6_loop.o uses the helper bpf_lwt_seg6_adjust_srh();
it will not be present if CONFIG_IPV6_SEG6_BPF is not specified.

Fixes: b061017f ("selftests/bpf: add realistic loop tests")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1590147389-26482-2-git-send-email-alan.maguire@oracle.com