libxfce4ui: update to git head

* in commit f30f9742e64ea5ec3b23d977b904f24799d773d6 the signature for
  xfce_shortcut_conflict_dialog was changed. This change is required for
  xfce4-setings HEAD. In the next patch of this series the development
  of a datetime setter for xfce4-settings will be introduced and this
  is put on top of xfce4-setings HEAD.
* there has been no release since 04.2012

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>