Add Gio to libxfce4panel gir includes
This fixes the following error when building xfce4-stopwatch-plugin:
Gio-2.0.gir:120281.7-120285.24: error: UnixSocketAddress' already contains a definition for
abstract'
Gio-2.0.gir:120071.7-120073.21: note: previous definition of `abstract'
was here