DataFormats/PatCandidates: export pat::Jet::numberOfDaughters() const

The following call, `pat::Jet::numberOfDaughters()` const, is being
used by `testCommonToolsUtil` from `CommonTools/Utils`. It was generated
with LOCAL binding, export it with WEAK binding.

Visible on GCC pre-4.9.0 IB: https://cmssdt.cern.ch/SDT/cgi-bin/buildlogs/slc6_amd64_gcc490/CMSSW_7_1_X_2014-02-14-0200/CommonTools/Utils

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>