scanner: Generate 'since' macros for requests as well

We already generate WL_[INTERFACE]_[REQUEST]_SINCE_VERSION macros for
events in the server protocol headers. Lets do the same for requests in
the client protocol headers as well.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: David FORT <contact@hardening-consulting.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>