include: Add the IDXGISwapChain3 interface.

For cross-compilation of demos. Ideally we'd just depend on the MinGW-w64
headers for these, but we can't depend on those being new enough to include
these interfaces.

include/unknown.idl

@@ -30,6 +30,9 @@ typedef IID *REFGUID;
 cpp_quote("#endif")
 
 cpp_quote("#if !defined(_WIN32)")
+typedef void *HWND;
+typedef void *HMODULE;
+
 typedef struct LUID
 {
     DWORD LowPart;