patch 8.1.0503: missing change to diff test

Problem:    Missing change to diff test. (Hirohito Higashi)
Solution:   Add the missing test function.
diff --git a/src/version.c b/src/version.c
index d2bed0e..bc22e73 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    503,
+/**/
     502,
 /**/
     501,