add dvdnav_get_number_of_audio_streams

dvdnav_get_number_of_audio_streams provides a way of getting the number of audio streams in vts titles or dvd menus

---

Similar to videolan/libdvdnav!32 (comment 311038) it allows to obtain the audio stream count without using the private VM. In Kodi the usage is pretty similar, we get the the audio stream count [1] [2] and then obtain the audio stream info via dvdnav_get_audio_attr [3].

Refs:

[1] https://github.com/xbmc/xbmc/blob/a80d1293b0579bbed7dd7d20304ba34a20519f78/xbmc/cores/VideoPlayer/DVDInputStreams/DVDInputStreamNavigator.cpp#L1118-L1142

[2] https://github.com/xbmc/xbmc/blob/a15445fe44b7c89296fcb7ee736ac813f2410254/xbmc/cores/VideoPlayer/VideoPlayer.cpp#L437-L454

[3] https://github.com/xbmc/xbmc/blob/a15445fe44b7c89296fcb7ee736ac813f2410254/xbmc/cores/VideoPlayer/DVDInputStreams/DVDInputStreamNavigator.cpp#L1105-L1128