patch 8.1.1367: can set 'modelineexpr' in modeline

Problem:    can set 'modelineexpr' in modeline.
Solution:   Add P_SECURE flag.
diff --git a/src/option.c b/src/option.c
index 8e62efc..39ef052 100644
--- a/src/option.c
+++ b/src/option.c
@@ -1889,7 +1889,7 @@
     {"modeline",    "ml",   P_BOOL|P_VIM,
 			    (char_u *)&p_ml, PV_ML,
 			    {(char_u *)FALSE, (char_u *)TRUE} SCTX_INIT},
-    {"modelineexpr", "mle",  P_BOOL|P_VI_DEF,
+    {"modelineexpr", "mle",  P_BOOL|P_VI_DEF|P_SECURE,
 			    (char_u *)&p_mle, PV_NONE,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},
     {"modelines",   "mls",  P_NUM|P_VI_DEF,