Add a "const" qualifier to unit_type::cfg_.

This was made possible by r56095, and reduces the number of blank attributes inserted
into the config. (Almost eliminates the blank attributes in unit_type configs, but
variations still get "inherited=" inserted if it was not specified.)


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56096 75b3775d-3801-0410-9e12-e2e43121521d