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

libxfce4panel/Makefile.am

@@ -145,7 +145,7 @@ if HAVE_INTROSPECTION
 introspection_sources = $(libxfce4panel_2_0_la_SOURCES)
 
 libxfce4panel-2.0.gir: libxfce4panel-2.0.la Makefile
-libxfce4panel_2_0_gir_INCLUDES = GObject-2.0 GLib-2.0 Gtk-3.0 libxfce4util-1.0
+libxfce4panel_2_0_gir_INCLUDES = GObject-2.0 GLib-2.0 Gio-2.0 Gtk-3.0 libxfce4util-1.0
 libxfce4panel_2_0_gir_CFLAGS = $(AM_CPPFLAGS)
 libxfce4panel_2_0_gir_LIBS = libxfce4panel-2.0.la
 libxfce4panel_2_0_gir_FILES = $(introspection_sources)