compilation failre with new mesh feature

[stevengj]

The CI is giving the error

geom.c:1964:30: error: unknown type name 'mesh_bvh_node'
 1964 | static void bvh_node_set_box(mesh_bvh_node *node, const geom_box *box) {

which seems to stem from #74. cc @Luochenghuang