fix const-correctness of write_sha1_file

fix const-correctness of write_sha1_file

These should take const buffers as input data, but zlib's
next_in pointer is not const-correct. Let's fix it at the
zlib level, though, so the cast happens in one obvious
place. This should be safe, as a similar cast is used in
zlib's example code for a const array.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>