Hi, Xfce4-panel 4.7 has moved all headers private, and only <libxfce4panel/libxfce4panel.h> should be included. The attached patch fixes the build for me. Thanks!
Created attachment 2975 Proposed patch
Merged the patch with the git repository: http://git.xfce.org/panel-plugins/xfce4-wmdock-plugin/