fdo#70393: move zlib to a subdir of external

Change-Id: I13dec52e6999d9c8aa0ba1f0c31ee8c36e1b6aac
Reviewed-on: https://gerrit.libreoffice.org/6556
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>