patch 9.0.0444: trying to declare g:variable gives confusing error

Problem:    Trying to declare g:variable gives confusing error.
Solution:   Give a better error message. (closes #11108)
diff --git a/src/proto/evalvars.pro b/src/proto/evalvars.pro
index 253af6d..b8b0053 100644
--- a/src/proto/evalvars.pro
+++ b/src/proto/evalvars.pro
@@ -13,6 +13,7 @@
 int get_spellword(list_T *list, char_u **pp);
 void prepare_vimvar(int idx, typval_T *save_tv);
 void restore_vimvar(int idx, typval_T *save_tv);
+int is_scoped_variable(char_u *name);
 char_u *eval_one_expr_in_str(char_u *p, garray_T *gap, int evaluate);
 list_T *heredoc_get(exarg_T *eap, char_u *cmd, int script_get, int vim9compile);
 void ex_var(exarg_T *eap);