Book2: Listing 29: include rtweekend.h

[estshorter]

rtweekend.h should be included as it uses random_double.

[rjkilpatrick]

It is missing from the book source,

raytracing.github.io/books/RayTracingTheNextWeek.html

Lines 1422 to 1425 in 7b92650

	#ifndef PERLIN_H
	#define PERLIN_H
	class perlin {

But can be found in the src directory:

raytracing.github.io/src/common/perlin.h

Lines 1 to 18 in 7b92650

	#ifndef PERLIN_H
	#define PERLIN_H
	//==============================================================================================
	// Originally written in 2016 by Peter Shirley <ptrshrl@gmail.com>
	//
	// To the extent possible under law, the author(s) have dedicated all copyright and related and
	// neighboring rights to this software to the public domain worldwide. This software is
	// distributed without any warranty.
	//
	// You should have received a copy (see file COPYING.txt) of the CC0 Public Domain Dedication
	// along with this software. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
	//==============================================================================================
	#include "rtweekend.h"
	class perlin {

It's also needed for point3 class etc.
I can submit a PR if needed