Added missing using statements.

svn path=/trunk/mcs/; revision=6056

mcs/class/corlib/System.Runtime.Remoting.Proxies/ProxyAttribute.cs

@@ -7,9 +7,11 @@
 //
 
 using System;
+using System.IO;
 using System.Runtime.Remoting;
 using System.Runtime.Remoting.Activation;
 using System.Runtime.Remoting.Contexts;
+using System.Runtime.Serialization;
 
 namespace System.Runtime.Remoting.Proxies {