Hoist cddl prebuild lib dependency definitions out of a MK_ZFS block

The compilation of several libraries under cddl/lib is not conditional
on MK_ZFS = "yes", so their dependency on libspl is not conditional
either. Unbreak buildworld when WITHOUT_ZFS is set.

Reported by: bz
Fixes: 9e9c651caceb ("cddl: fix missing ZFS library dependencies")
MFC after: 1 week