wayland-server: hide wl_priv_signal from doxygen

Fix this set of warnings appearing three times during a build:

/home/pq/git/wayland/src/wayland-server.c:1868: warning: class
`wl_priv_signal' for related function `wl_priv_signal_init' is not
documented.
/home/pq/git/wayland/src/wayland-server.c:1884: warning: class
`wl_priv_signal' for related function `wl_priv_signal_add' is not
documented.
/home/pq/git/wayland/src/wayland-server.c:1899: warning: class
`wl_priv_signal' for related function `wl_priv_signal_get' is not
documented.

Our Wayland docbook don't include private things, so make sure these do
not end up there. This removes the mention of wl_priv_signal_emit from
the Server API docbook. I have no idea why the other functions did not
appear there.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>