patch 8.2.3390: included xdiff code is outdated

Problem:    Included xdiff code is outdated.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/xprepare.h b/src/xdiff/xprepare.h
index b67b3b2..947d9fc 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) */