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/dict.c b/src/dict.c
index 1097e58..b2c3fbc 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -957,7 +957,7 @@
  * "what" == 1: list of values
  * "what" == 2: list of items
  */
-    void
+    static void
 dict_list(typval_T *argvars, typval_T *rettv, int what)
 {
     list_T	*l2;