ProcStats : Use mutex lock to make fetch() thread safe.

Whilst investigating a possible memory issue using the Memory Auditor, I discovered the information printed was garbled if multiple threads where used.

The issue was the ProcStats::fetch method which was not thread safe. This MR fixes that with a simple mutex lock.

I also did a drive-by cleanup and code modernisation.

FYI @clemenci

GaudiAud/src/ProcStats.cpp

@@ -18,6 +18,7 @@
 #endif
 
 #include "ProcStats.h"
+#include <memory>
 
 #if defined( __linux__ ) or defined( __APPLE__ )
 #  include <iostream>