Fixed CopyClassFields helper to prefer complete declarations.

src/Generator/Library.cs

@@ -229,16 +229,13 @@ namespace CppSharp
         public static void CopyClassFields(this ASTContext context, string source,
             string destination)
         {
-            foreach (var @class in context.FindClass(source))
-            {
-                foreach (var dest in context.FindClass(destination))
-                {
+            var @class = context.FindCompleteClass(source);
+            var dest = context.FindCompleteClass(destination);
+
             dest.Fields.AddRange(@class.Fields);
             foreach (var field in dest.Fields)
                 field.Namespace = dest;
         }
-            }
-        }
 
         #endregion