include l1prefire weight for all eras

Previously there was no l1 prefiring correction for 2018. However, there are still some muon prefiring issues in 2018 (see here: https://twiki.cern.ch/twiki/bin/viewauth/CMS/L1PrefiringWeightRecipe) and the corrections (including systematic uncertainties) are in nanoV9.

This just makes sure we access those for 2018 as we do for other eras.