Need to add R__ADD_INCLUDE_PATH(/include)

Author: preghenella

MC/config/PWGHF/external/generator/GeneratorHF.C

@@ -3,6 +3,8 @@
 // Example of an implementation of an event generator
 // that provides HF signals for embedding in background
 
+R__ADD_INCLUDE_PATH($PYTHIA_ROOT/include)
+
 #include "Pythia8/Pythia.h"
 
 namespace o2 {