patch 8.1.1891: functions used in one file are global

Problem:    Functions used in one file are global.
Solution:   Add "static". (Yegappan Lakshmanan, closes #4840)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 77e925d..77d3943 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -20,7 +20,7 @@
 
 static int linelen(int *has_tab);
 static void do_filter(linenr_T line1, linenr_T line2, exarg_T *eap, char_u *cmd, int do_in, int do_out);
-
+static int not_writing(void);
 static int check_readonly(int *forceit, buf_T *buf);
 static void delbuf_msg(char_u *name);
 static int help_compare(const void *s1, const void *s2);
@@ -2456,7 +2456,7 @@
  * Check the 'write' option.
  * Return TRUE and give a message when it's not set.
  */
-    int
+    static int
 not_writing(void)
 {
     if (p_write)