Forward-declare struct bpf_redir_neigh in bpf_helper_defs.h to avoid
compiler warning about unknown structs.

Fixes: ba452c9e ("bpf: Fix bpf_redirect_neigh helper api to support supplying nexthop")
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Toke Høiland-Jørgensen <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]