utils: Add doxygen for wayland-util.h
utils: Add doxygen for wayland-util.h

This is needed for doxygen to generate output for macro definitions, such
as wl_container_of, that are contained by this file. Classes like
wl_list would be documented regardless.

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