add a NOTE comment to the object safety test so that it passes

Author: mikeyhew

src/test/compile-fail/arbitrary-self-types-not-object-safe.rs

@@ -35,6 +35,7 @@ fn make_foo() {
     //~| ERROR E0038
     //~| NOTE method `foo` has a non-standard `self` type
     //~| NOTE the trait `Foo` cannot be made into an object
+    //~| NOTE required because of the requirements on the impl of `std::ops::CoerceUnsized<std::boxed::Box<Foo>>` for `std::boxed::Box<usize>`
 }
 
 fn make_bar() {