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/XmlElement.cpp b/xmlserializer/XmlElement.cpp
index bda3973..106bfdc 100644
--- a/xmlserializer/XmlElement.cpp
+++ b/xmlserializer/XmlElement.cpp
@@ -32,6 +32,9 @@
 #include <stdlib.h>
 #include <sstream>
 
+using std::string;
+using std::ostringstream;
+
 CXmlElement::CXmlElement(_xmlNode* pXmlElement) : _pXmlElement(pXmlElement)
 {
 }