Fixed Array.Empty for .NET 3

IPA.Loader/Loader/PluginManifest.cs

@@ -6,6 +6,10 @@ using System;
 using System.Collections.Generic;
 using AlmostVersionConverter = IPA.JsonConverters.AlmostVersionConverter;
 using Version = SemVer.Version;
+#if NET3
+using Net3_Proxy;
+using Array = Net3_Proxy.Array;
+#endif
 
 namespace IPA.Loader
 {