Fixed compilation of BVH_Example (required for make doc)

unsupported/doc/examples/BVH_Example.cpp

@@ -6,9 +6,7 @@ using namespace Eigen;
 typedef AlignedBox<double, 2> Box2d;
 
 namespace Eigen {
-    namespace internal {
         Box2d bounding_box(const Vector2d &v) { return Box2d(v, v); } //compute the bounding box of a single point
-    }
 }
 
 struct PointPointMinimizer //how to compute squared distances between points and rectangles