Minor fix to generator_pythia8_LF.C (#1731)

1. fixing : error: use of undeclared identifier 'nlohmann'
2. fixing : error: reference to 'Pythia' is ambiguous

(cherry picked from commit ab8d3ed)

Author: hkoley

MC/config/PWGLF/pythia8/generator_pythia8_LF.C

@@ -23,8 +23,8 @@
 #include <fstream>
 #include "Generators/GeneratorPythia8Param.h"
 #include "Generators/DecayerPythia8Param.h"
-#include <nlohmann/json.hpp>
 #endif
+#include <nlohmann/json.hpp>
 #include "generator_pythia8_longlived.C"
 
 using namespace Pythia8;
@@ -445,8 +445,8 @@ class GeneratorPythia8LF : public o2::eventgen::GeneratorPythia8
 
   std::vector<ConfigContainer> mGunConfigs;           // List of gun configurations to use
   std::vector<ConfigContainer> mGunConfigsGenDecayed; // List of gun configurations to use that will be decayed by the generator
-  Pythia pythiaObjectSignal;                          // Signal collision generator
-  Pythia pythiaObjectMinimumBias;                     // Minimum bias collision generator
+  Pythia8::Pythia pythiaObjectSignal;                          // Signal collision generator
+  Pythia8::Pythia pythiaObjectMinimumBias;                     // Minimum bias collision generator
 };
 
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