libctf, types: allow ctf_type_reference of dynamic slices

libctf, types: allow ctf_type_reference of dynamic slices

One spot was missed when we rejigged ctf_update into ctf_serialize and
allowed all operations on dynamic containers: ctf_type_reference of
slices.  A dynamic slice's vlen state is stored in the dtu_slice member,
so fetch it from there.

libctf/
	* ctf-types.c (ctf_type_reference): Add support for dynamic slices.