tools: ynl-gen-c: don't require -o argument

Without -o the tool currently crashes, but it's not marked
as required. The only thing we can't do without it is to
generate the correct #include for user source files, but
we can put a placeholder instead.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20241206113100.89d35bf124d6.I9228fb704e6d5c9d8e046ef15025a47a48439c1e@changeid


Signed-off-by: Jakub Kicinski <kuba@kernel.org>