Build-Depend on gnome-shell to avoid uninstallable binaries

@aerostitch s390x is a release architecture where gnome-shell is not available.

Once you upload this change, gnome-shell-pomodoro won't build on s390x. Then ask the Debian Release Team to remove gnome-shell-pomodoro on s390x from Testing. This should enable gnome-shell-pomodoro to migrate to Testing.