change temporary_values_conf.global_speed_add to add

temporary_values_conf.global_speed_add is set to latest for now

this setting is useless however, since the correct value is last, not latest (defined in engine/Entity.lua:912

But, changing anything regarding global speed will affect a lot about game balance and make most global speed items/talents useless, so the safe way is just leave it as it is, just give it acorrect value (add)