patch 8.2.1699: build failure due to missing error message

Problem:    Build failure due to missing error message.
Solution:   Add error message.
diff --git a/src/errors.h b/src/errors.h
index 9513a42..d12e97f 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -266,4 +266,6 @@
 	INIT(= N_("E1120: Cannot change dict"));
 EXTERN char e_cannot_change_dict_item[]
 	INIT(= N_("E1121: Cannot change dict item"));
+EXTERN char e_variable_is_locked_str[]
+	INIT(= N_("E1122: Variable is locked: %s"));
 #endif