patch 8.2.3119: compiler warning for unused argument

Problem:    Compiler warning for unused argument.
Solution:   Add UNUSED.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 34eac69..bdfb570 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -522,7 +522,7 @@
     return &t_list_dict_any;
 }
     static type_T *
-ret_list_items(int argcount, type_T **argtypes UNUSED)
+ret_list_items(int argcount UNUSED, type_T **argtypes UNUSED)
 {
     return &t_list_list_any;
 }