patch 8.1.0538: evaluating a modeline might invoke using a shell command

Problem:    Evaluating a modeline might invoke using a shell command. (Paul
            Huber)
Solution:   Set the sandbox flag when setting options from a modeline.
diff --git a/src/version.c b/src/version.c
index 29488c2..8569417 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    538,
+/**/
     537,
 /**/
     536,