[xml] small fix needed for win32

["Vakoc, Mark" <Mark_Vakoc jdedwards com>]

The utitlites in libxml2 (xmllint, test*) and libxslt (xsltproc) won't build
on win32 straight out of the box due to linking errors of unresolved items.

In libxml.h, which is included by the libxml utilities contains the line
#define IN_LIBXML 

This should only be defined when building libxml2, not the utilites that
link against it.

This messes up the definition of LIBXML_DLL_IMPORT, defined as
#ifndef LIBXML_DLL_IMPORT
#if defined(_MSC_VER) && !defined(IN_LIBXML) && !defined(LIBXML_STATIC)
#define LIBXML_DLL_IMPORT __declspec(dllimport)
#else
#define LIBXML_DLL_IMPORT
#endif
#endif

The second definition (blank) is used instead of the first.

Removing the #define IN_LIBXML works on win32 because it is defined in the
libxml2 project as a preprocessor definition when building libxml2 itself.
Not sure if that would mess up the other platforms builds

The same applies to libxslt with IN_LIBXSLT