Remove private symbols from Vorbis shared libs

[yugr]

I recently noticed that Vorbis shlibs export several symbols which are
not present in it's public header (for a total of 85 symbols, see
attach). Would it make sense to filter them via -fvisibility=hidden
and -Wl,--version-script? I attached a draft patch.

The issue was found using ShlibVisibilityChecker tool.

Private_symbols.txt
0001-Hide-private-symbols.patch.txt

As a side note, it seems that instructions on filing bugs at Vorbis site are outdated ("Bugs" at https://xiph.org/vorbis/ lead to Trac instead of Github).