git-zlib: cast away potential constness of `next_in` pointer

git-zlib: cast away potential constness of `next_in` pointer

The `struct git_zstream::next_in` variable points to the input data and
is used in combination with `struct z_stream::next_in`. While that
latter field is not marked as a constant in zlib, it is marked as such
in zlib-ng. This causes a couple of compiler errors when we try to
assign these fields to one another due to mismatching constness.

Fix the issue by casting away the potential constness of `next_in`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>