tools: ynl-gen: print prototypes for recursive stuff

We avoid printing forward declarations and prototypes for most
types by sorting things topologically. But if structs nest we
do need the forward declarations, there's no other way.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231213231432.2944749-9-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>