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

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Similar to !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