upgrade to zlib-1.2.8

Change-Id: I67db2bfb386f1cc7ffeb92ec12a64e12e4552160
Reviewed-on: https://gerrit.libreoffice.org/9315
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>