scanner: Make emit_structs more explicit

"is_interface" is a really terrible name for the client or server
variants, and instead of checking whether we were passed the requests or
the events, just pass an argument through.

Reviewed-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>