Do not generate ToString override for value types - not handled properly in generated code yet.

src/Generator/Passes/ObjectOverridesPass.cs

@@ -100,6 +100,11 @@ namespace CppSharp
             if (!VisitDeclaration(@class))
                 return false;
 
+            // We can't handle value types yet
+            // The generated code assumes that a NativePtr is available
+            if (@class.IsValueType)
+                return false;
+
             foreach (var method in @class.Methods)
             {
                 if (!IsInsertionOperator(method))
@@ -127,9 +132,6 @@ namespace CppSharp
                 break;
             }
 
-            if (@class.IsValueType)
-                return false;
-
             var methodEqualsParam = new Parameter
             {
                 Name = "object",