Killall is not installed in the VM

The commit 75060eee (on the branch feature/stretch) introduce the command killall, see the diff here:

https://labs.riseup.net/code/projects/tails/repository/revisions/75060eeee5f87e1aac472ebeb4596b2a7e3df2f3/diff/vagrant/provision/assets/build-tails

However killall is not installed on the VM, hence the failure:

+ sudo killall -q gpg-agent
sudo: killall: command not found
rake aborted!

Either it needs to be installed, either pkill should be use instead.

Related issues

• Related to #12527 (closed)

Original created by @arnaud on 12537 (Redmine)