If API cannot be copyrighted (Google vs. Oracle), I'm not sure headers that provide access to a DLL that aren't "GPL compatible" really matter in the legalese equation.
Headers that provide the bare necessities to use a DLL aren't really source code.
I am not a lawyer, and all that. Still, I find some interpretations of the GPL to be outlandish --- and not everyone's interpretation of the GPL matters, only the upstream of the source base you are using. 95% percent of GPL source code contributing people care about the spirit of the GPL, not uber fringe cases and most of those that care about uber fringe cases are not generally actual producers of GPL source code (ie. having an opinion of the GPL does not qualify one to have influence or say, it takes someone using your code to have a voice). Still if someone is like the one major VLC source code contributor and has an ultra-hardline opinion, that opinion better be heeded when it comes to using the VLC source code

The night is young. How else can I annoy the world before sunsrise?

Inquisitive minds want to know ! And if they don't -- well like that ever has stopped me before ..