fix feature API in disc.c
fix feature API in disc.c

The API was how I wanted it in the includes,
but not in the implementation in disc.c.
The wrong name was chosen and since we didn't
use that part of the API, no error was logged.

phw/libdiscid,phw/libdiscid,metabrainz/libdiscid,metabrainz/libdiscid