Add missing build package in Readme

[lstolcman]

Hi

I was building the project under raspberry pi os bullseye (debian for raspberry pi), but got an error:

lukasz@rpi-fat:~/dump1090 $ ./prepare-build.sh bullseye
Updating changelog for bullseye backport build
./prepare-build.sh: 48: dch: not found

debchange (abbreviation dch) is located in devscripts package. Therefore, I propose to add the package to the list of requirements.