patch 8.2.4171: cannot invoke option function using autoload import

Problem:    Cannot invoke option function using autoload import.
Solution:   Expand the import to an autoload function name. (closes #9578)
diff --git a/src/proto/evalvars.pro b/src/proto/evalvars.pro
index 487f88b..df4fb16 100644
--- a/src/proto/evalvars.pro
+++ b/src/proto/evalvars.pro
@@ -103,5 +103,6 @@
 void put_callback(callback_T *cb, typval_T *tv);
 void set_callback(callback_T *dest, callback_T *src);
 void copy_callback(callback_T *dest, callback_T *src);
+void expand_autload_callback(callback_T *cb);
 void free_callback(callback_T *callback);
 /* vim: set ft=c : */