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/list.c b/src/list.c
index fc34867..a0c710d 100644
--- a/src/list.c
+++ b/src/list.c
@@ -53,7 +53,7 @@
  * Just before removing an item from a list: advance watchers to the next
  * item.
  */
-    void
+    static void
 list_fix_watch(list_T *l, listitem_T *item)
 {
     listwatch_T	*lw;