Remove warnings in disk.cpp and rectangle.cpp

Author: Speierers

src/shapes/disk.cpp

@@ -186,6 +186,7 @@ class Disk final : public Shape<Float, Spectrum> {
         Float local_x = cache[0];
         Float local_y = cache[1];
 #else
+        ENOKI_MARK_USED(cache);
         Ray3f ray_    = m_world_to_object.transform_affine(ray);
         Float t       = -ray_.o.z() / ray_.d.z();
         Point3f local = ray_(t);

src/shapes/rectangle.cpp

@@ -216,6 +216,7 @@ class Rectangle final : public Shape<Float, Spectrum> {
         Float local_x = cache[0];
         Float local_y = cache[1];
 #else
+        ENOKI_MARK_USED(cache);
         Ray3f ray     = m_world_to_object.transform_affine(ray_);
         Float t       = -ray.o.z() * ray.d_rcp.z();
         Point3f local = ray(t);