WinSDK: extract System.MCX submodule

Currently this header gets included into `WinSDK.WinSock2` via `windows.h`

Author: egorzhdan

stdlib/public/Platform/winsdk.modulemap

@@ -280,6 +280,11 @@ module WinSDK [system] {
       header "winioctl.h"
       export *
     }
+
+    module MCX {
+      header "mcx.h"
+      export *
+    }
   }
 
   module OLE32 {