Add -dock to dzen command

[Ongy]

Description

Adds the -dock argument to the dzen sapwner in XMonad.Hooks.DynamicLog, which is required for dzen to set struts.

Checklist

• I've read CONTRIBUTING.md

• I tested my changes with xmonad-testing

• I updated the CHANGES.md file

[mention-bot]

@Ongy, thanks for your PR! By analyzing the history of the files in this pull request, we identified @shachaf, @byorgey and @spencerjanssen to be potential reviewers.

[srid]

I believe this should be merged. Otherwise dzen simply doesn't work, at least in NixOS.