Include `pythia8 +fastjet`

Fastjet is required for events generated by https://github.com/bspage912/eicSimuBeamEffects/tree/headonTestJin

spack.yaml

@@ -17,7 +17,7 @@ spack:
           +fftw +fortran +gdml +http +mlp +pythia8 
           +root7 +tmva +vc +xrootd +ssl ^mesa+opengl 
           -llvm -osmesa
-    - pythia8@8303
+    - pythia8@8303 +fastjet
     - hepmc3@3.2.2 +python +rootio 
     - stow@2.3.1
     - cairo@1.16.0 +fc+ft+X+pdf+gobject