The libreadline-gplv2-dev package has been removed from the current Debian Sid repositories because it was old and unmaintained.
Based on this: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=980504 we should be able to switch over to using libeditreadline-dev, assuming a gplv2-compatible readline is still needed. That package is installed on the updated Sid builders on buildbot.askmonty.org.
In any case the current compile dependency in debian/control of libreadline-gplv2-dev means compiling on Sid is currently broken and will be broken in future releases of Debian and Ubuntu.
It's probably also worth noting that Sid now includes MariaDB 10.5, so any fix needs to go at least to there and higher.