Add Debian bullseye

For things like webkit development we want the very latest so
let's allow for Debian bullseye builds as well.

This needs the security suites added when buster becomes stable.

conf/dists/bullseye

+mirror="http://deb.debian.org/debian"
+apt_sources="deb http://deb.debian.org/debian bullseye main
+deb http://debug.mirrors.debian.org/debian-debug/ bullseye-debug main
+"