scripts/bpf: Emit an #error directive known types list needs updating

Make the compiler report a clear error when bpf_helpers_doc.py needs
updating rather than rely on the fact that Clang fails to compile
English:

../../../lib/bpf/bpf_helper_defs.h:2707:1: error: unknown type name 'Unrecognized'
Unrecognized type 'struct bpf_inet_lookup', please add it to known types!

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191016085811.11700-1-jakub@cloudflare.com