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/globals.h b/src/globals.h
index 8cc9385..b6356a3 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1788,6 +1788,7 @@
 EXTERN char e_lock_unlock[]	INIT(= N_("E940: Cannot lock or unlock variable %s"));
 EXTERN char e_const_req_value[] INIT(= N_("E1021: const requires a value"));
 EXTERN char e_type_req[]	INIT(= N_("E1022: type or initialization required"));
+EXTERN char e_declare_global[]	INIT(= N_("E1016: Cannot declare a global variable: %s"));
 #endif
 #if defined(FEAT_GUI) || defined(FEAT_TERMGUICOLORS)
 EXTERN char e_alloc_color[]	INIT(= N_("E254: Cannot allocate color %s"));