Fix for failing dataflow build

Hybrid generator includes Pythia8, so when the dataflow build happens, without this fix, the Pythia8 header is loaded. It's not found due to the nature of the dataflow build. This fix prevents the Hybrid generator loading if Pythia8 is not found and it includes also HepMC3, given that a similar issue might arise with that generator as well in the future.

Author: jackal1-66

Generators/CMakeLists.txt

@@ -24,8 +24,6 @@ o2_add_library(Generators
                        src/GeneratorTGenerator.cxx
                        src/GeneratorExternalParam.cxx
                        src/GeneratorFromFile.cxx
-                       src/GeneratorHybrid.cxx
-                       src/GeneratorHybridParam.cxx
                        src/GeneratorFromO2KineParam.cxx
                        src/GeneratorFileOrCmd.cxx
                        src/GeneratorFileOrCmdParam.cxx
@@ -50,6 +48,8 @@ o2_add_library(Generators
                        $<$<BOOL:${HepMC3_FOUND}>:src/GeneratorHepMC.cxx>
                        $<$<BOOL:${HepMC3_FOUND}>:src/GeneratorHepMCParam.cxx>
                        $<$<BOOL:${HepMC3_FOUND}>:src/AODToHepMC.cxx>
+                       $<$<AND:$<BOOL:${pythia_FOUND}>,$<BOOL:${HepMC3_FOUND}>>:src/GeneratorHybrid.cxx>
+                       $<$<AND:$<BOOL:${pythia_FOUND}>,$<BOOL:${HepMC3_FOUND}>>:src/GeneratorHybridParam.cxx>
                PUBLIC_LINK_LIBRARIES FairRoot::Base O2::SimConfig O2::CommonUtils O2::DetectorsBase O2::ZDCBase
                                      O2::SimulationDataFormat ${pythiaTarget} ${hepmcTarget}
                                      FairRoot::Gen