ctfmerge: Fix missing pthread_cond_init()

This does not appear to matter on FreeBSD or Linux, but when building an
amd64 kernel on macOS I was seeing infinite loops in ctfmerge.
It turns out the loop in wip_save_work() was looping forever due to
pthread_cond_wait() always returning -EINVAL.

Reviewed By: markj, brooks
Differential Revision: https://reviews.freebsd.org/D25973