Fix uninitialized value in Cycles BVH after recent changes

Found by asan, unknown if it actually caused an issue.

intern/cycles/bvh/params.h

@@ -142,6 +142,7 @@ class BVHParams {
 
     top_level = false;
     bvh_layout = BVH_LAYOUT_BVH2;
+    use_compact_structure = true;
     use_unaligned_nodes = false;
 
     num_motion_curve_steps = 0;