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/menu.c b/src/menu.c
index 29f58c7..4096a05 100644
--- a/src/menu.c
+++ b/src/menu.c
@@ -29,6 +29,7 @@
 static void free_menu_string(vimmenu_T *, int);
 static int show_menus(char_u *, int);
 static void show_menus_recursive(vimmenu_T *, int, int);
+static char_u *menu_name_skip(char_u *name);
 static int menu_name_equal(char_u *name, vimmenu_T *menu);
 static int menu_namecmp(char_u *name, char_u *mname);
 static int get_menu_cmd_modes(char_u *, int, int *, int *);
@@ -1557,7 +1558,7 @@
  * element.  Any \ and ^Vs are removed from the current element.
  * "name" may be modified.
  */
-    char_u *
+    static char_u *
 menu_name_skip(char_u *name)
 {
     char_u  *p;