winnt.h: fix dllimport and visiblity attributes on cygwin

[Steven Edwards]

This one also fixes a metric butt ton of warnings when compiling most dlls

changlog:
fix dllimport and visiblity attributes on cygwin

-- 
Steven Edwards

"There is one thing stronger than all the armies in the world, and
that is an idea whose time has come." - Victor Hugo
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