Unexported struct fields which are pointers are not deep copied

## Reproducible Example

https://go.dev/play/p/xa2-zmGBrU9

## Description

Refer to the example above.

```
type Address struct {
	Street string
	City   string
}

type Person struct {
	Name string
	Age  int
	addr *Address
}
```

If a `struct` has an unexported field and if it is a pointer then `DeepCopy` of this unexported pointer does not happen.

