Fix -Wreorder flagging

[13steinj]

If using -Werror=reorder via some cmake build or otherwise, this gets flagged.

Might want to -Wall -Wextra -Werror or more in the cmake as well? I'd do it myself but I don't follow the whole "duplicate the lib in the BOOST_COBALT_IS_ROOT" case and/or where to move the copyright notice. Also don't know any policies regarding how to set those flags as part of a boost project.