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/parameter/FormattedSubsystemObject.cpp b/parameter/FormattedSubsystemObject.cpp
index 63502c7..2da7deb 100644
--- a/parameter/FormattedSubsystemObject.cpp
+++ b/parameter/FormattedSubsystemObject.cpp
@@ -35,6 +35,8 @@
 
 #define base CSubsystemObject
 
+using std::string;
+
 CFormattedSubsystemObject::CFormattedSubsystemObject(
         CInstanceConfigurableElement* pInstanceConfigurableElement)
     : base(pInstanceConfigurableElement)