net: atm: bring back zatm uAPI

Jiri reports that linux-atm does not build without this header.
Bring it back. It's completely dead code but we can't break
the build for user space :(

Reported-by: Jiri Slaby <jirislaby@kernel.org>
Fixes: 052e1f01

 ("net: atm: remove support for ZeitNet ZN122x ATM devices")
Link: https://lore.kernel.org/all/8576aef3-37e4-8bae-bab5-08f82a78efd3@kernel.org/
Link: https://lore.kernel.org/r/20220810164547.484378-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>