patch 8.1.0506: modeline test fails when run by root

Problem:    Modeline test fails when run by root.
Solution:   Set 'modeline' for the test. (James McCoy, closes #3592)
diff --git a/src/version.c b/src/version.c
index 5da6027..54b9a6c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    506,
+/**/
     505,
 /**/
     504,