bpf: Add struct bpf_redir_neigh forward declaration to BPF helper defs

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 <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20201028181204.111241-1-andrii@kernel.org