patch 8.2.2920: still a way to shadow a builtin function

Problem:    Still a way to shadow a builtin function. (Yasuhiro Matsumoto)
Solution:   Check the key when using extend(). (issue #8302)
diff --git a/src/proto/dict.pro b/src/proto/dict.pro
index dbfdcdc..7789d4c 100644
--- a/src/proto/dict.pro
+++ b/src/proto/dict.pro
@@ -13,6 +13,7 @@
 void dictitem_remove(dict_T *dict, dictitem_T *item);
 void dictitem_free(dictitem_T *item);
 dict_T *dict_copy(dict_T *orig, int deep, int copyID);
+int dict_wrong_func_name(dict_T *d, typval_T *tv, char_u *name);
 int dict_add(dict_T *d, dictitem_T *item);
 int dict_add_number(dict_T *d, char *key, varnumber_T nr);
 int dict_add_bool(dict_T *d, char *key, varnumber_T nr);