fix a compilation pb with clang (it's actually surprising gcc did not complain)
fix a compilation pb with clang (it's actually surprising gcc did not complain)

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