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/xprepare.h b/src/xdiff/xprepare.h
index 947d9fc..b67b3b2 100644
--- a/src/xdiff/xprepare.h
+++ b/src/xdiff/xprepare.h
@@ -31,4 +31,4 @@
 
 
 
-#endif /* #if !defined(XPREPARE_H) */
+#endif // #if !defined(XPREPARE_H)