add core::clustering header for clustering_serv
add core::clustering header for clustering_serv

mathn/jubatus,elkingtonmcb/jubatus,roselleebarle04/jubatus,rimms/jubatus,jubatus/jubatus,gintenlabo/jubatus,jubatus/jubatus,rimms/jubatus,mathn/jubatus,jubatus/jubatus,roselleebarle04/jubatus,gintenlabo/jubatus,elkingtonmcb/jubatus