Disable 'Unused' warning in Gtk/Interop/Native.cs

src/Gtk/Avalonia.Gtk3/Interop/Native.cs

@@ -1,4 +1,5 @@
-using System;
+#pragma warning disable 649
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;