You just need the development package libxfcegui4-dev. However it seems that plugin requires libxfcegui4 version 4.1 or greater, and the current version in debian testing is 4.0.6. You'll have to update almost whole xfce4 just to get this plugin working.
The xfce 4.2 brach is stable now so official debian packages for it should appear pretty soon. There already are some unofficial builds such as this
. Just in case you decide to upgrade; Xfce4.2 stores config files in ~/.config instead of ~/.xfce4. You'll have to transfer old config files manually to the new location.