Book2: Listing 15: include hittable_list.h

[estshorter]

hittable_list.h should be included as bvh_node uses hittable_list.

[estshorter]

@hollasch
I can submit PRs for this issue, #692 and #694, if needed.

[hollasch]

Thank you! What's your full name, so I can add you to the acknowledgments?

Please submit a PR for the dev-patch branch.

[estshorter]

Thanks.
My full name is Shota Kawajiri.

[hollasch]

Added.

[hollasch]

Fixed.