patch 8.2.4330: Vim9: no error if script imports itself

Problem:    Vim9: no error if script imports itself.
Solution:   Give an error when a script imports itself.
diff --git a/src/errors.h b/src/errors.h
index ecd4eb9..c66d26d 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -2802,7 +2802,8 @@
 	INIT(= N_("E1086: Function reference invalid"));
 EXTERN char e_cannot_use_index_when_declaring_variable[]
 	INIT(= N_("E1087: Cannot use an index when declaring a variable"));
-// E1088 unused
+EXTERN char e_script_cannot_import_itself[]
+	INIT(= N_("E1088: Script cannot import itself"));
 EXTERN char e_unknown_variable_str[]
 	INIT(= N_("E1089: Unknown variable: %s"));
 EXTERN char e_cannot_assign_to_argument[]