[Cogl] Make gtk-doc not warn on external types

[Damien Lespiau]

Hey,

Just so people are aware and don't spend time looking at that, I've
submitted a patch to gtk-doc to ignore external types that cannot be
cross-referenced

  https://bugzilla.gnome.org/show_bug.cgi?id=690941

For Cogl:
  
  --ignore-list=HWND,MSG,SDL_Event,XEvent,Display

reduces the warnings we have even further.

-- 
Damien