[WinSDK] Modularize <guiddef.h>

This fixes modularization errors that arise when importing a C++ header that contains `#include <guiddef.h>`, which might hijack this header from the WinSDK module where it belongs.

Author: egorzhdan

stdlib/public/Platform/winsdk.modulemap

@@ -285,6 +285,11 @@ module WinSDK [system] {
     requires cplusplus
   }
 
+  module GUID {
+    header "guiddef.h"
+    export *
+  }
+
   module Internationalization {
     module WinNLS {
       header "winnls.h"