Add curl to the building prerequisites
tools/download_file.py calls curl and so building Gerrit on a
fresh system without curl (E.g.: fresh debian buster) fails with
ERROR: An error occurred during the fetch of repository 'impl-log4j': failed [/usr/bin/python, [...]/gerrit/tools/download_file.py, [...] could not invoke curl: [Errno 2] No such file or directory is curl installed?
As curl is obviously required, we list it as prerequisite.