netlink: specs: rt-route: add C naming info

Add properties needed for C codegen to match names with uAPI headers.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250410014658.782120-7-kuba@kernel.org


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