Fixed mean reduction leading to unresolved symbol.
Fixed mean reduction leading to unresolved symbol.

madlib/eigen_backup,mjbshaw/Eigen,mjbshaw/Eigen,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen,rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,mjbshaw/Eigen,rotorliu/eigen,madlib/eigen_backup