patch 8.2.0514: several global functions are used in only one file

Problem:    Several global functions are used in only one file.
Solution:   Make the functions static. (Yegappan Lakshmanan, closes #5884)
diff --git a/src/list.c b/src/list.c
index a7f4d40..b532e28 100644
--- a/src/list.c
+++ b/src/list.c
@@ -20,6 +20,8 @@
 // List heads for garbage collection.
 static list_T		*first_list = NULL;	// list of all lists
 
+static void list_free_item(list_T *l, listitem_T *item);
+
 /*
  * Add a watcher to a list.
  */
@@ -311,7 +313,7 @@
  * Free a list item, unless it was allocated together with the list itself.
  * Does not clear the value.  Does not notify watchers.
  */
-    void
+    static void
 list_free_item(list_T *l, listitem_T *item)
 {
     if (l->lv_with_items == 0 || item < (listitem_T *)l
@@ -1225,7 +1227,7 @@
     rettv->vval.v_string = ga.ga_data;
 }
 
-    void
+    static void
 list_remove(typval_T *argvars, typval_T *rettv, char_u *arg_errmsg)
 {
     list_T	*l;