Ensure proper initialisation of the G4GammaNuclearXS data before even…

… before G4-v-11.2.0.

The G4GammaNuclearXS data are uploaded in its constructor from G4-v-11.2.0
while it is done in the BuildPhysicsTable interface before G4-v-11.2.0.
This modification works fine with all G4 versions (from 11.0.0).

G4HepEm/G4HepEmInit/src/G4HepEmGammaInit.cc

@@ -63,6 +63,7 @@ void InitGammaData(struct G4HepEmData* hepEmData, struct G4HepEmParameters* /*he
   if (nullptr == xs) {
     xs = new G4GammaNuclearXS();
   }
+  xs->BuildPhysicsTable(*g4PartDef);
   G4CrossSectionDataStore hadGNucXSDataStore;
   hadGNucXSDataStore.AddDataSet(xs);