patch 8.2.0878: no reduce() function

Problem:    No reduce() function.
Solution:   Add a reduce() function. (closes #5481)
diff --git a/src/proto/list.pro b/src/proto/list.pro
index c276874..77183af 100644
--- a/src/proto/list.pro
+++ b/src/proto/list.pro
@@ -51,4 +51,5 @@
 void f_insert(typval_T *argvars, typval_T *rettv);
 void f_remove(typval_T *argvars, typval_T *rettv);
 void f_reverse(typval_T *argvars, typval_T *rettv);
+void f_reduce(typval_T *argvars, typval_T *rettv);
 /* vim: set ft=c : */