patch 8.0.1689: no tests for xxd

Problem:    No tests for xxd.
Solution:   Add a test. (Christian Brabandt)
diff --git a/src/version.c b/src/version.c
index 63c274f..196d61c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -763,6 +763,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1689,
+/**/
     1688,
 /**/
     1687,