win32: Fix incorrect usage of interfaces (value type instead of reference type)

D interfaces are reference types. Thus, "IUnknown*" means different things in C and D headers. "IUnknown**" should almost never appear in D code, as it indicates a pointer to pointer to pointer.

git-svn-id: http://svn.dsource.org/projects/bindings/trunk@408 e3080b08-b6e0-0310-b0a6-cbd5e3aa05fd