patch 8.2.3376: Vim9: no warning that "@r" does not do anything

Problem:    Vim9: no warning that "@r" does not do anything.
Solution:   Give a "no effect" error. (closes #8779)
diff --git a/src/proto/ex_eval.pro b/src/proto/ex_eval.pro
index f5037c9..a3be429 100644
--- a/src/proto/ex_eval.pro
+++ b/src/proto/ex_eval.pro
@@ -12,6 +12,7 @@
 void discard_current_exception(void);
 void catch_exception(except_T *excp);
 void report_make_pending(int pending, void *value);
+int cmd_is_name_only(char_u *arg);
 void ex_eval(exarg_T *eap);
 void ex_if(exarg_T *eap);
 void ex_endif(exarg_T *eap);