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

[jhb]

Summary

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.

[imp]

Should these also be upstreamed?

[emaste]

There's a bigger question of what to do w/ zlib upstream with K&R, so it seems fine to make this change in our tree now and separately try to tackle all of that together.