patch 8.2.1024: Vim9: no error for using "let g:var = val"

Problem:    Vim9: no error for using "let g:var = val".
Solution:   Add an error.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 2ac7a0a..54f47b9 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -4871,8 +4871,7 @@
 		dest = dest_global;
 		if (is_decl)
 		{
-		    semsg(_("E1016: Cannot declare a global variable: %s"),
-									 name);
+		    semsg(_(e_declare_global), name);
 		    goto theend;
 		}
 	    }