zsh-completion: Add missing flag to compadd

zsh-completion: Add missing flag to compadd

This fixes an issue with zsh completion where certain words were not added to the
list of matches, but incorrectly interpreted as flags or options.

By passing the "--" flag, compadd is notified that all following arguments should
be considered for completion and not interpreted as flags or options for compadd.

Details can be found in the compadd documentation:
http://zsh.sourceforge.net/Doc/Release/Completion-Widgets.html#Completion-Builtin-Commands

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>