Fixed compilation error on clang 3.1.
Fixed compilation error on clang 3.1.

stat class' private member stat_val is used in mixable_stat so made protected.

gintenlabo/jubatus,kumagi/jubatus_core,kmaehashi/jubatus,roselleebarle04/jubatus,rimms/jubatus_core,Asuka52/jubatus,gintenlabo/jubatus_core,rimms/jubatus_core,elkingtonmcb/jubatus,gintenlabo/jubatus_core,mathn/jubatus,rimms/jubatus,rimms/jubatus_core,kumagi/jubatus_core,gintenlabo/jubatus_core,gintenlabo/jubatus,kmaehashi/jubatus_core,kmaehashi/jubatus_core,kmaehashi/jubatus,gintenlabo/jubatus_core,kumagi/jubatus_core,kumagi/jubatus_core,kmaehashi/jubatus_core,jubatus/jubatus,jubatus/jubatus,elkingtonmcb/jubatus,Asuka52/jubatus,rimms/jubatus,jubatus/jubatus,kmaehashi/jubatus_core,roselleebarle04/jubatus,rimms/jubatus_core,mathn/jubatus