stand/zlib: Document the upstream issue behind NO_DEPRECATED_NON_PROTOTYPE

The zlib project has issue https://github.com/madler/zlib/issues/633 to
document its continued use of old K&R-style function definitions.

Suggested by: delphij@
Sponsored by: Netflix

(cherry picked from commit 6b574b3ba99af4ae81d4c619057e737e67523907)