Add missing copyright header

Author: chrisseaton

src/main/java/org/truffleruby/core/ReferenceProcessingService.java

@@ -1,3 +1,12 @@
+/*
+ * Copyright (c) 2018 Oracle and/or its affiliates. All rights reserved. This
+ * code is released under a tri EPL/GPL/LGPL license. You can use it,
+ * redistribute it and/or modify it under the terms of the:
+ *
+ * Eclipse Public License version 1.0, or
+ * GNU General Public License version 2, or
+ * GNU Lesser General Public License version 2.1.
+ */
 package org.truffleruby.core;
 
 import java.lang.ref.ReferenceQueue;