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/SelectionCriterionRule.cpp b/parameter/SelectionCriterionRule.cpp
index 7f9e912..6f350c3 100644
--- a/parameter/SelectionCriterionRule.cpp
+++ b/parameter/SelectionCriterionRule.cpp
@@ -37,6 +37,8 @@
 
 #define base CRule
 
+using std::string;
+
 const CSelectionCriterionRule::SMatchingRuleDescription CSelectionCriterionRule::_astMatchesWhen[CSelectionCriterionRule::ENbMatchesWhen] = {
     { "Is", true },
     { "IsNot", true },