Remove using std::XXX from headers

This is a bad practice to have using in headers because it pollutes the
namespace of any user of that header.
diff --git a/xmlserializer/XmlDocSource.cpp b/xmlserializer/XmlDocSource.cpp
index 5e53a81..35a8f4e 100644
--- a/xmlserializer/XmlDocSource.cpp
+++ b/xmlserializer/XmlDocSource.cpp
@@ -33,6 +33,8 @@
 #include <libxml/xmlschemas.h>
 #include <stdlib.h>
 
+using std::string;
+
 // Schedule for libxml2 library
 bool CXmlDocSource::_bLibXml2CleanupScheduled;