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/MappingContext.cpp b/parameter/MappingContext.cpp
index 852f89f..b627051 100644
--- a/parameter/MappingContext.cpp
+++ b/parameter/MappingContext.cpp
@@ -32,6 +32,8 @@
 #include <string.h>
 #include <stdlib.h>
 
+using std::string;
+
 CMappingContext::CMappingContext(uint32_t uiNbItemTypes) : _pstItemArray(new CMappingContext::SItem[uiNbItemTypes]), _uiNbItemTypes(uiNbItemTypes)
 {
     // Clear items