tools: ynl-gen: factor out free_needs_iter for a struct

Instead of walking the entries in the code gen add a method
for the struct class to return if any of the members need
an iterator.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250429154704.2613851-3-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>