fix sort_devs_by_distance in btl/openib

The distances as returned by hwloc_get_whole_distance_matrix_by_type are typ float.
This patch handle all distances as float.

cmr=v1.8.2:reviewer=miked
fix sort_devs_by_distance in btl/openib

no need to #include <math.h> ...

cmr=v1.8.2:reviewer=miked:ticket=4759