Fix marshalling return type (#71)

***UPDATE_DEPENDENTS***

Author: josesimoes

source/MetadataProcessor.Core/nanoSkeletonGenerator.cs

@@ -133,7 +133,7 @@ private void GenerateStubs()
 
                                 newMethod.ReturnType = m.MethodReturnType.ReturnType.ToNativeTypeAsString();
 
-                                newMethod.MarshallingReturnType = newMethod.ReturnType;
+                                newMethod.MarshallingReturnType = m.MethodReturnType.ReturnType.ToCLRTypeAsString();
 
                                 declaration.Append($"{m.Name}");
                                 declaration.Append("( ");