[AD] declaration of WinMain with gcc and C++

[Sven S. <svs...@st...>]

I _think_ the attached patch fixes a typo in
include/allegro/platform/alwin.h (missing return type in function
declaration). I'll leave committing to someone who knows this WinMain
stuff better than me (if it's correct).
-- 
Sven Sandberg    svs...@st...    home.student.uu.se/svsa1977