wmllint: ids in [time_area] are not associated to units most of the time

The exception is when the [time_area] action is part of an event and
it contains an id-based SUF inside an SLF, but that should be rare
enough.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56343 75b3775d-3801-0410-9e12-e2e43121521d