patch 8.1.0823: not sufficient testing of xxd

Problem:    Not sufficient testing of xxd.
Solution:   Add some more test coverage.
diff --git a/src/version.c b/src/version.c
index 4d7730c..b952291 100644
--- a/src/version.c
+++ b/src/version.c
@@ -788,6 +788,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    823,
+/**/
     822,
 /**/
     821,