xproto: add explicit align-pad in struct Setup

This pad is defined in the spec:
http://cgit.freedesktop.org/xorg/proto/xproto/tree/specs/encoding.xml?id=xproto-7.0.28#n875

And it is also implemented in the Server:
http://cgit.freedesktop.org/xorg/xserver/tree/dix/dispatch.c?id=xorg-server-1.17.99.901#n532



Signed-off-by: Christian Linhart <chris@demorecorder.com>
Reviewed-by: Peter Harris <pharris@opentext.com>