Allow mocking functions with unknown size type bounds

[HampusMat]

A quick change to allow for mocked functions to have type bounds with unknown sizes.

For example:

use mockall::mock;

trait Foo {}

trait Bar {
    fn do_something<T>(&self) -> u32
    where
        T: 'static + ?Sized;
}

mock! {
    Foobar {}

    impl Bar for Foobar {
        fn do_something<T>(&self) -> u32
        where
            T: 'static + ?Sized;
    }
}

let mut foobar_mock = MockFoobar::new();

foobar_mock
    .expect_do_something::<dyn Foo>()
    .returning(|| 120);

let num = foobar_mock.do_something::<dyn Foo>();

println!("{num}");

[asomers]

This is a nice little fix. Don't forget to add it to the "Fixed" section of the CHANGELOG!