patch 8.2.3390: included xdiff code is outdated

Problem:    Included xdiff code is outdated.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/xdiffi.h b/src/xdiff/xdiffi.h
index 467a1e8..8f1c7c8 100644
--- a/src/xdiff/xdiffi.h
+++ b/src/xdiff/xdiffi.h
@@ -61,4 +61,4 @@
 int xdl_do_histogram_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 		xdfenv_t *env);
 
-#endif // #if !defined(XDIFFI_H)
+#endif /* #if !defined(XDIFFI_H) */