zlib: Use NO_WDEPRECATED_NON_PROTOTYPE

Also add it to kern.mk so it's available for module builds.

Sponsored by: Netflix
Noticed by: mjg
Fixes: b9f235ba3178
Differential Revision: https://reviews.freebsd.org/D38550

(cherry picked from commit b80185c267243a57bbc3e03fd2b47680672d6050)