[racket] Building Racket on windows: missing ffi.h

Looks I accidentally replied privately to Eli

> I've added them too.  It's questionable whether the first two should
> be added to the "textual" distribution since they build the gracket
> too, but it's probably helpful to have them in anyway.  Also, sgc is
> not usually used, but I guess that it should be included since the
> corresponding "src/racket/sgc" is in too.

Thanks. Btw racket.sln refers to sgc.vcproj and just does not build
without the sgc directory.