python,python3: add zlib/host to HOST_BUILD_DEPENDS

python,python3: add zlib/host to HOST_BUILD_DEPENDS

This should fix the zlibmodule build on the host side.

Usually, if zlib is not found, Python/Python3 builds fine
without it, but there are some cases where the Python/Python3
interpreter on the host-side requires zlib to run.

At the moment, zlib does not have a host-build.
This should be available when this PR gets merged:
https://github.com/lede-project/source/pull/1329
[ or a similar one that contains host-build support for zlib ].

In the meantime, this change can go into Python/Python3.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>