HPCC-20270 Address non-xmllib build break

- Adds xpathprocess.hpp include statement in xmllib_unsupported.cpp

Signed-off-by: Rodrigo Pastrana <rodrigo.pastrana@lexisnexis.com>

system/xmllib/xmllib_unsupported.cpp

@@ -2,6 +2,7 @@
 #include "xslprocessor.hpp"
 #include "xmlvalidator.hpp"
 #include "xmlerror.hpp"
+#include "xpathprocessor.hpp"
 
 XMLLIB_API IXmlDomParser* getXmlDomParser()
 {