gdbusactiongroup: Add prototype for g_dbus_action_group_sync()

Even private functions that are actually called across compilation
units should have prototypes.  For g_dbus_action_group_sync(), create
one in gdbusactiongroup-private.h

https://bugzilla.gnome.org/show_bug.cgi?id=687385

gio/gactiongroupexporter.c

@@ -45,7 +45,7 @@
  * g_dbus_action_group_get() to obtain a #GDBusActionGroup.
  */
 
-G_GNUC_INTERNAL GVariant *
+static GVariant *
 g_action_group_describe_action (GActionGroup *action_group,
                                 const gchar  *name)
 {