Revert "Remove protocol/wayland.dtd"

This reverts commit 06fb8bd3.

Having a DTD hooked up gives an indication of what we expect the protocol to
be, which is a clearer documentation than the current "whatever scanner.c
manages to parse".

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>