shm: Move deprecated function to the bottom of the file

In wayland-server.c we group the deprecated functions and
disable doxygen for them.  Do that here too.

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>