patch 8.1.2389: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/xdiff/xmacros.h b/src/xdiff/xmacros.h
index 2809a28..1167ebb 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)