Small optimization: unit_type_data::add_advancement() will only have an effect the first
time it is called for a given unit_type. So only call it once per unit_type (instead of
once per unit_type per build status).


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56111 75b3775d-3801-0410-9e12-e2e43121521d