server: use void* instead of function pointer for wl_object.implementation
server: use void* instead of function pointer for wl_object.implementation

This is needed to make C++ programs that include wayland-server.h build:
C++ does not allow conversions from data pointers to function pointers.

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