Platform: add dxgidebug.h to _DXGI module

The debug header is used for enumeration of certain DXGI interfaces
related to debugging of the pipeline.  Add this to gain access to the
interfaces and some of the global GUIDs associated with it.

Author: compnerd

stdlib/public/Platform/winsdk.modulemap

@@ -184,6 +184,7 @@ module WinSDK [system] {
     // separate module is meant to augment the uncovered portions only.
     module _DXGI {
       header "../shared/dxgi1_6.h"
+      header "dxgidebug.h"
       export *
 
       link "dxgi.lib"