[TaglibExtractor] Include vnd.audible.aaxc audio books in supported types

The format is just another ISOBMFF derived format, i.e. metatdata can
be extracted in the same way. The major difference is the encrypted
content stream, which is not of concern here.

Also see commit 0975ed45 ("[TaglibExtractor] Add support for Audible
"Enhanced Audio" audio books").