maked xmlXPathNodeSetAddNs() function public for XMLSec performance

* xpath.c include/libxml/xpathInternals.h: maked xmlXPathNodeSetAddNs()
          function public for XMLSec performance optimizations
diff --git a/xpath.c b/xpath.c
index 4f4e771..99c3d2e 100644
--- a/xpath.c
+++ b/xpath.c
@@ -1588,7 +1588,7 @@
  *
  * add a new namespace node to an existing NodeSet
  */
-static void
+void
 xmlXPathNodeSetAddNs(xmlNodeSetPtr cur, xmlNodePtr node, xmlNsPtr ns) {
     int i;