build: Fix regenerating libxfce4ui.h after configure options change

Add a dependency on config.h to ensure that libxfce4ui.h is regenerate whenever ./configure options change. Otherwise, once generated the header is never invalidated, and the X11 variant could be installed with --disable-x11 or the other way around.

This is particularly a problem because X11-enabled header is included in the source distribution, effectively preventing it from being updated even if the package is initially configured with --disable-x11.

Originally reported as https://bugs.gentoo.org/924164.