server: use the right function pointer type in wl_signal_get
server: use the right function pointer type in wl_signal_get

use the wl_notify_func type, and not void *, or else wl_signal_get
will not be usable by a c++ plugin because it will not cast
void * to a function pointer.

eyolfson/wayland,bitfactor/wayland,bitfactor/wayland,eyolfson/wayland,bitfactor/wayland,sir-murray/wayland,mchalupa/wayland,sir-murray/wayland,abooij/wayland,eyolfson/wayland,tulcod/wayland,sir-murray/wayland,mchalupa/wayland,tulcod/wayland,mchalupa/wayland,tulcod/wayland,abooij/wayland,abooij/wayland