patch 8.2.1968: Vim9: has() assumes a feature does not change dynamically

Problem:    Vim9: has() assumes a feature does not change dynamically.
Solution:   Check whether a feature may change dynamically. (closes #7265)
diff --git a/src/proto/evalfunc.pro b/src/proto/evalfunc.pro
index c649b86..562a7c8 100644
--- a/src/proto/evalfunc.pro
+++ b/src/proto/evalfunc.pro
@@ -16,6 +16,7 @@
 void execute_redir_str(char_u *value, int value_len);
 void execute_common(typval_T *argvars, typval_T *rettv, int arg_off);
 void f_has(typval_T *argvars, typval_T *rettv);
+int dynamic_feature(char_u *feature);
 void mzscheme_call_vim(char_u *name, typval_T *args, typval_T *rettv);
 void range_list_materialize(list_T *list);
 float_T vim_round(float_T f);