add @unused.

Author: VladPodilnyk

compat/src/main/scala-2.11_2.12/scala/annotation/unused.scala

@@ -0,0 +1,15 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.annotation
+
+final class unused extends deprecated("unused", "unused")