apply patch suggested here: https://sft.its.cern.ch/jira/browse/ROOT-8481

core/multiproc/inc/PoolUtils.h

@@ -17,13 +17,14 @@
 #include "TObject.h"
 #include <vector>
 
+
+namespace ROOT {
 //////////////////////////////////////////////////////////////////////////
 ///
 /// This namespace contains pre-defined functions to be used in
 /// conjuction with TExecutor::Map and TExecutor::MapReduce.
 ///
 //////////////////////////////////////////////////////////////////////////
-namespace ROOT {
    namespace ExecutorUtils {
      //////////////////////////////////////////////////////////////////////////
      /// Merge collection of TObjects.