avoid calling gaudi_install_headers twice

gaudi_add_library(... PUBLIC_HEADERS ...) already calls gaudi_install_headers()

cf: https://twiki.cern.ch/twiki/bin/view/LHCb/GaudiCMakeConfiguration#Installing_Headers_without_a_lib

(assuming this was not intentionally called multiple times, in which case I would suggest adding a comment pointing that out).