subsequent runs of debian/autobake-deb.sh are not idempotent

If you run the debian/autobake-deb.sh multiple times on the same source directory it will fail with the error:

dh: error: debian/control has a duplicate entry for mariadb-plugin-columnstore

This happens because the columnstore section of the autobake-deb.sh script has a simple `cat` statement to add the mariadb-plugin-columnstore package config and it doesn't check if the config has already been added to debian/control, it just blindly adds an additional package config block to the end of debian/control every time it is run.

This won't ever show up in buildbot because we're always running debian/autobake-deb.sh on a fresh directory. But it becomes a pain when trying to debug package builds locally because you have to remember to remove the columnstore package config from the end of debian/control prior to re-running the script.

Running the script should be idempotent, meaning back-to-back runs should have the same result assuming no changes have been made to the source tree. The way to fix would be to either:

1. Add a check to the columnstore section of the script to not run the cat command unless the mariadb-plugin-columnstore package config is missing from debian/control
   ...or...
2. Add the mariadb-plugin-columnstore package config to debian/control so that it doesn't have to be added at compile time