tools/net/ynl: Rename _fixed_header_size() to _struct_size()

Refactor the _fixed_header_size() method to be _struct_size() so that
naming is consistent with _encode_struct() and _decode_struct().

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-9-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>