fix warning with gcc 4.2
fix warning with gcc 4.2

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