[df] Request dictionary for mergeable THnSparseD

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

(cherry picked from commit 2192821)

Author: vepadulano

tree/dataframe/inc/LinkDef.h

@@ -54,6 +54,7 @@
 #pragma link C++ class ROOT::Detail::RDF::RMergeableValue<TH2D>+;
 #pragma link C++ class ROOT::Detail::RDF::RMergeableValue<TH3D>+;
 #pragma link C++ class ROOT::Detail::RDF::RMergeableValue<THnD>+;
+#pragma link C++ class ROOT::Detail::RDF::RMergeableValue<THnSparseD>+;
 #pragma link C++ class ROOT::Detail::RDF::RMergeableValue<TGraph>+;
 #pragma link C++ class ROOT::Detail::RDF::RMergeableValue<TGraphAsymmErrors>+;
 #pragma link C++ class ROOT::Detail::RDF::RMergeableValue<TStatistic>+;