RootUtils: Fix compilation with gcc14.

Missing include.

Control/RootUtils/src/WithRootErrorHandler.cxx

 /*
- * Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration.
+ * Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration.
  */
 /**
  * @file RootUtils/src/WithRootErrorHandler.cxx
@@ -11,6 +11,7 @@
 
 #include "RootUtils/WithRootErrorHandler.h"
 #include <vector>
+#include <cstdlib>
 
 
 namespace {