patch 8.2.1667: local function name cannot shadow a global function name

Problem:    Local function name cannot shadow a global function name.
Solution:   Ignore global functions when checking a script-local or scoped
            function name. (closes #6926)
diff --git a/src/proto/userfunc.pro b/src/proto/userfunc.pro
index e6acc18..4e3f2db 100644
--- a/src/proto/userfunc.pro
+++ b/src/proto/userfunc.pro
@@ -11,6 +11,7 @@
 char_u *fname_trans_sid(char_u *name, char_u *fname_buf, char_u **tofree, int *error);
 ufunc_T *find_func_even_dead(char_u *name, int is_global, cctx_T *cctx);
 ufunc_T *find_func(char_u *name, int is_global, cctx_T *cctx);
+int func_is_global(ufunc_T *ufunc);
 void copy_func(char_u *lambda, char_u *global);
 int call_user_func_check(ufunc_T *fp, int argcount, typval_T *argvars, typval_T *rettv, funcexe_T *funcexe, dict_T *selfdict);
 void save_funccal(funccal_entry_T *entry);