The xdmcp handling isn't really OS specific, and only few sites actually need
to call it, so at least it's prototypes are better off in some separate header.
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1311>
Enrico Weigelt, metux IT consult