book nextWeek: add missing include <algorithm>

Author: hollasch

books/RayTracingTheNextWeek.html

@@ -764,6 +764,9 @@
 This yields:
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
+    #include <algorithm>
+    ...
+
     bvh_node::bvh_node(std::vector<hittable*>& objects, int n, double time0, double time1) {
         int axis = random_int(0,2);
         auto comparator = (axis == 0) ? box_x_compare

src/TheNextWeek/bvh.h

@@ -29,6 +29,7 @@ class bvh_node : public hittable  {
         virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const;
         virtual bool bounding_box(double t0, double t1, aabb& output_box) const;
 
+    public:
         hittable *left;
         hittable *right;
         aabb box;