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
2 jobs for !2 with fix-gir in 4 minutes and 5 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Build
passed #666
build

00:01:37

 
  Distcheck
passed #667
distcheck

00:02:28