[xml] making xmlXPathNodeSetAddNs() public

[Aleksey Sanin <aleksey aleksey com>]

Hi, Daniele!

Welcome back from vacation :) Hope you've had good time :)
I have one more request for "opening" some internal LibXML2
functions. I am doing some performance optimization in XMLSec
and I found that I need static xmlXPathNodeSetAddNs() function
from xpath.c file. Will you mind if I make it public (simple changes
in xpath.c and xpathInternals.h)?

Thanks,

Aleksey.