Implement PeGetSubSystem() routine

xtldr/modules/pecoff/includes/pecoff.h

@@ -17,6 +17,10 @@ EFI_STATUS
 PeGetEntryPoint(IN PPECOFF_IMAGE_CONTEXT Image,
                 OUT PVOID *EntryPoint);
 
+EFI_STATUS
+PeGetSubSystem(IN PPECOFF_IMAGE_CONTEXT Image,
+               OUT PUSHORT SubSystem);
+
 EFI_STATUS
 PeLoadImage(IN PEFI_FILE_HANDLE FileHandle,
             IN LOADER_MEMORY_TYPE MemoryType,