Add libz-dev to required build packages in Debian documentation.
This is apparently not installed by default in Ubuntu 20.04 as it was in prior versions.
<execute if="{[os-type-is-debian]}" user="root" pre="y">
<exe-cmd>
apt-get install make gcc libpq-dev libssl-dev libxml2-dev pkg-config
- liblz4-dev libzstd-dev libbz2-dev
+ liblz4-dev libzstd-dev libbz2-dev libz-dev
</exe-cmd>
<exe-cmd-extra>-y 2>&amp;1</exe-cmd-extra>
</execute>
