[xml] Report libxslt bug (minor issue)

[Hao Hu <ihaohu gmail com>]

Basically, in libexslt/exslt.c

The
#include "exsltconfig.h"
should be
#include <libexslt/exsltconfig.h>

Else the issue is:

When trying to compile outside of the libxslt source tree.
It won't use the new exsltconfig.h generated by libtool.

Cheers,