how to avoid the xmlns:xi on each include statement

Hi all!

    I have the following dummy problem: I need to <xi:include> lots of 
files into my manual.docbook however each line currently looks as

<xi:include href="aboutdialoginfo.docbook" 
xmlns:xi="http://www.w3.org/2001/XInclude"/>

is there a way I can avoid the xmlns:xi attribute ?
I have a big number of <xi:include> and it will be a lot of redundancy 
if I must put the xmlns:xi attribute in each tag.

I have tried to put the xmlns:xi attribute at the root of my docbook 
element <book> but then xmllint complains it's not a valid attribute for 
docbook DTD:

book: validity error : No declaration for attribute xmlns:xi of element book

I am using the following command line for validation:

xmllint --xinclude --postvalid --noent --nonet --noout 
docbook/manual.docbook

Any hint?

Thanks!
Francesco