add full5x5 shower variables to reco::Photon

Correcting an inconsistency which entered in 71x and propagated to 72x.

For history purposes:
In CMSSW_7_0_X the full5x5 shower variables were added to pat::Electron and pat::Photon so they would be easily accessible in the MiniAOD (the need for these variables arose after the reco part of the release was closed and they were added during the miniaod development stage)

In 71X the full5x5 variables were added to the reco::GsfElectron, but due to some oversight/miscommunication never to the reco::Photon.  The changes adding the full5x5 variables to the pat::Electron AND photon classes were nevertheless intentionally kept out of 71x, resulting in these variables being nowhere accessible for photons in 72x MiniAOD.  (Well they could be recomputed from the rechits, but this is a nasty regression compared to 70x from an end user point of view.)

@lgray @arizzi @gpetruc