libproc: link against libctf if MK_CTF != no instead of MK_CDDL != no

Logic prior to this change would incorrectly try linking when MK_CDDL != no,
instead of MK_CTF != no, which could result in the library and the tests being
broken if/when MK_CTF == no and MK_CDDL != no (an uncommon, but possible
combination with today's build knobs).

This change updates the conditional to correctly track the value of MK_CTF, which
in turn is properly toggled to no if/when MK_CDDL == no as it's a dependent build
knob.

This [niche] build bug has been present in FreeBSD since 2014.

MFC after: 1 week