help: include git-zlib.h to print zlib version

help: include git-zlib.h to print zlib version

In 41f1a8435a (git-compat-util: move include of "compat/zlib.h" into
"git-zlib.h", 2025-01-28) some code was refactored to enable easier
linking against zlib-ng.

This removed `zlib.h` being indirectly included in `help.c`. As this
file uses `ZLIB_VERSION` to print the version number of zlib when
running git-version(1) with `--build-options`, this resulted in a
regression.

Include `git-zlib.h` directly into `help.c` to print zlib version
information. This brings back the zlib version in the output of
`git version --build-options`.

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