Created attachment 9798 PATCH - fix gtk2 removal xfce4-panel is not buildable. As of the patch to remove gtk2 from xfce4-panel (https://git.xfce.org/xfce/xfce4-panel/commit/?id=19b2977543f3f72b1f23853878355fbbbf2e6fa8), there is one leftover piece that needs to be removed. Attached is a patch to fix the issue.
Thanks for the patch but I also noticed a day later and pushed the fix to master.