[df] Request dictionary for mergeable RCutFlowReport

This allows proper serialization in case of calling Report operation in distributed mode.

(cherry picked from commit 066fd6e)

Author: vepadulano

tree/dataframe/inc/LinkDef.h

@@ -64,6 +64,7 @@
 #pragma link C++ class TNotifyLink<ROOT::Internal::RDF::RNewSampleFlag>;
 #pragma link C++ class ROOT::RDF::RCutFlowReport;
 #pragma link C++ class ROOT::RDF::TCutInfo;
+#pragma link C++ class ROOT::Detail::RDF::RMergeableValue<ROOT::RDF::RCutFlowReport>+;
 
 #endif