fontselect: silence warnings about discarding const

fontselect: silence warnings about discarding const

Fixing this properly involves constifying ASS_FontProviderMetaData
and refactoring code that allocates and frees strings stored in it.
This seems easy on the surface but turns out to be nontrivial when
you actually try to do it. This may still be done at a later date,
but for now, just add explicit casts.