Platform: add dxgi1_6 to the Direct3D module

There is no usermode header which has the DXGI1.6 interfaces included
unfortunately.  This adds the interfaces to the module which is required
for IDXGIAdapter4 interface.

(cherry picked from commit 4084f7a)

Author: compnerd

stdlib/public/Platform/winsdk.modulemap

@@ -165,6 +165,9 @@ module WinSDK [system] {
       header "d3d11_2.h"
       header "d3d11_3.h"
       header "d3d11_4.h"
+
+      header "../shared/dxgi1_6.h"
+
       export *
 
       link "d3d11.lib"