libxfce4ui: fix order of inherits

* we need gtk-doc before xfce-git, because gtk-doc needs configure.in or configure.ac
  and the repository contains only configure.ac.in which is transformed into
  configure.ac by autogen which is prepended to do_configure by xfce-git (so
  this .bbclass needs to appear in inherit after gtk-doc to prepend autogen.sh
  before gtkdocize call.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>