fixed one more c14n + namespaces corner case from new Merlin's test suite

* c14n.c include/libxml/c14n.h: fixed one more c14n + namespaces                corner case from new Merlin's test suite and added a callback                   that will be used to improve xmlsec performance
diff --git a/include/libxml/c14n.h b/include/libxml/c14n.h
index 43e5304..75ace8a 100644
--- a/include/libxml/c14n.h
+++ b/include/libxml/c14n.h
@@ -43,6 +43,8 @@
  *    ctxt->replaceEntities = 1;
  *    ...
  */
+
+
 int 		xmlC14NDocSaveTo	(xmlDocPtr doc,
 					 xmlNodeSetPtr nodes,
 					 int exclusive,
@@ -66,6 +68,21 @@
 					 int compression);
 
 
+/**
+ * This is the core C14N function
+ */
+typedef int (*xmlC14NIsVisibleCallback)	(void* user_data, 
+					 xmlNodePtr node,
+					 xmlNodePtr parent);
+
+int 		xmlC14NExecute		(xmlDocPtr doc,
+					 xmlC14NIsVisibleCallback is_visible_callback,
+					 void* user_data,				 
+					 int exclusive,
+					 xmlChar **inclusive_ns_prefixes,
+					 int with_comments, 
+					 xmlOutputBufferPtr buf);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */