Add missing copyright notice

OpenRA.Mods.Common/ActorInitializer.cs

@@ -1,4 +1,15 @@
-using System;
+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System;
 using OpenRA.Traits;
 
 namespace OpenRA.Mods.Common