shm: Deprecate wl_shm_buffer_create()

From irc:
<pq> it creates a wl_buffer object in a way that no client can ever
     access the storage.

So, let's replace it with return NULL; and mark it with attribute
deprecated in the header.

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