stand/zlib: Zlib still uses K&R function definitions

So add ${NO_WDEPRECATED_NON_PROTOTYPE} to the CFLAGS of those
files. This can be removed when we import a zlib that's free of this
anachronism.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D37516

(cherry picked from commit 983a18021decd68fbcf5d14ccd60c3d66dac1c6a)