helpers: add struct bpf_redir_neigh forward declaration

This avoids compilation warning if `struct bpf_redir_neigh` is not provided by
other kernel headers.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

src/bpf_helper_defs.h

@@ -6,6 +6,7 @@ struct bpf_sk_lookup;
 struct bpf_perf_event_data;
 struct bpf_perf_event_value;
 struct bpf_pidns_info;
+struct bpf_redir_neigh;
 struct bpf_sock;
 struct bpf_sock_addr;
 struct bpf_sock_ops;