share/mk: Add opencsd to the list of private libs

Reported by: Sarah Walker <Sarah.Walker2@arm.com>
Reviewed by: emaste
Fixes: 1c52229528e7 ("lib: Move libopencsd to /usr/lib and make private")
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D53217

(cherry picked from commit 82778970eeb24f1249945612d05923725f9baa4f)