zlib crc32: Add prototypes for recently-added static functions.

This appeases -Wstrict-prototypes with GCC. Using a separate
prototype using the OF macro seems to be consistent with the rest of
this file which still supports pre-ANSI C.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D36755

(cherry picked from commit 7550e242ab4c118ba48171577ccadf81ba739847)