PileUpMT: Fix placement of using declarations.

Don't put using declarations in the global namespace in headers.

Control/PileUpMT/src/PileUpMTAlg.cxx

@@ -120,6 +120,7 @@ StatusCode PileUpMTAlg::add_subevt(
 }
 
 StatusCode PileUpMTAlg::initialize() {
+  using namespace std::chrono;
   ATH_MSG_DEBUG("Initializing " << name() << "...");
   if (m_writeTrace) {
     m_pileupTrace.init(