patch 8.2.3390: included xdiff code is outdated

Problem:    Included xdiff code is outdated.
Solution:   Sync with xdiff in git 2.33. (Christian Brabandt, closes #8431)
diff --git a/src/xdiff/xmacros.h b/src/xdiff/xmacros.h
index 1167ebb..2809a28 100644
--- a/src/xdiff/xmacros.h
+++ b/src/xdiff/xmacros.h
@@ -51,4 +51,4 @@
 } while (0)
 
 
-#endif // #if !defined(XMACROS_H)
+#endif /* #if !defined(XMACROS_H) */