maked xmlXPathNodeSetAddNs() function public for XMLSec performance

* xpath.c include/libxml/xpathInternals.h: maked xmlXPathNodeSetAddNs()
          function public for XMLSec performance optimizations
diff --git a/include/libxml/xpathInternals.h b/include/libxml/xpathInternals.h
index a75862e..80b6b43 100644
--- a/include/libxml/xpathInternals.h
+++ b/include/libxml/xpathInternals.h
@@ -478,6 +478,9 @@
 						 xmlNodePtr val);
 void              xmlXPathNodeSetAddUnique	(xmlNodeSetPtr cur,
 						 xmlNodePtr val);
+void		  xmlXPathNodeSetAddNs		(xmlNodeSetPtr cur, 
+						 xmlNodePtr node, 
+						 xmlNsPtr ns);
 void              xmlXPathNodeSetSort		(xmlNodeSetPtr set);
 
 void		  xmlXPathRoot			(xmlXPathParserContextPtr ctxt);