protocol: spell out that we're using linux/input-event-codes.h button codes

Because we already rely on it in the callers anyway. This is a retrofit, which
is not ideal but I'm not sure any compositor out there uses anything else.
Might as well define it.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jonas Ã…dahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
1 file changed