Add missing license

backend/mirai-console/src/main/kotlin/net/mamoe/mirai/console/permission/PermissibleIdentifier.kt

@@ -1,3 +1,12 @@
+/*
+ * Copyright 2019-2020 Mamoe Technologies and contributors.
+ *
+ * 此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
+ * Use of this source code is governed by the GNU AFFERO GENERAL PUBLIC LICENSE version 3 license that can be found via the following link.
+ *
+ * https://github.com/mamoe/mirai/blob/master/LICENSE
+ */
+
 @file:Suppress("unused")
 
 package net.mamoe.mirai.console.permission