xcb: Remove unnecessary padding fields

Some structures allow access to the padding fields from the C structures. Since
these don't do anything except make the memory be set out correctly there's no
reason to access them from Vala.

https://bugzilla.gnome.org/show_bug.cgi?id=730094
xcb: Missing EnterNotifyEvent, LeaveNotifyEvent, KeymapNotifyEvent, VisibilityNotifyEvent, ReparentNotifyEvent, GravityNotifyEvent, CirculateNotifyEvent, PropertyNotifyEvent, SelectionNotifyEvent, ColormapNotifyEvent, MappingNotifyEvent classes

https://bugzilla.gnome.org/show_bug.cgi?id=730097