patch 8.1.0540: may evaluate insecure value when appending to option

Problem:    May evaluate insecure value when appending to option.
Solution:   Set the secure flag when changing an option that was previously
            set insecurely.  Also allow numbers for the characters from
            'spelllang' that are used for LANG.vim.

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    540,
 /**/
     539,
 /**/