irc/irssi: Backport buildfix for Capsicum in -CURRENT

After base 000321bab7be and base e4c36b7aa952 with CAPSICUM enabled
build fails to find nvlist_create() .
Backport upstream commit 7baefc3315e9dc11c8a3a787a3b6c6888c2b590a to
fix this issue

PR: 265397
Reported by: vishwin
Reviewed by: David O'Rourke <dor.bsd@xm0.uk> (maintainer)
Tested by: kp