libctf, create: add explicit casts for variables' and slices' types

libctf, create: add explicit casts for variables' and slices' types

This is technically unnecessary -- the compiler is quite capable of
doing the range reduction for us -- but it does mean that all
assignments of a ctf_id_t to its final uint32_t representation now have
appropriate explicit casts.

libctf/
	* ctf-create.c (ctf_serialize): Add cast.
	(ctf_add_slice): Likewise.