Add some defines in winnt.h.

[Corinna Vinschen]

Chris Faylor wrote:
> 
> Since Mumit isn't around, I've taken the liberty of approving this patch.
> It looks fine to me.
> 
> This is conditional on Mumit's approval when he returns, of course.

Oops, I have also patched w32api by adding some needed
defines in wincrypt.h two or three weeks ago. We must not
forget them since they are needed to compile Cygwin.

Corinna