vamp-plugin-sdk: Update to 2.9.0

Version 2.9, 2019-11-13 (maintenance release)

  * Fix non-thread-safe behaviour in PluginAdapter. Plugins built
    using the adapter classes in version 2.8 or earlier cannot safely
    be used simultaneously across threads with other instances of
    themselves or of other plugins in the same library (i.e. shared
    object). Hosts have been required to provide synchronisation for
    such cases. Version 2.9 introduces synchronisation in the plugin,
    making this usage safe. Unfortunately this does not make host code
    safe when using older plugin builds, as the problem and its fix
    are in the plugin side of the SDK. Caution is still required, but
    this fix does allow updated plugin builds to avoid problems with
    some existing hosts
  * Change required C++ language standard from C++98 to C++11. This
    is because of the use of std::mutex in the above fix


(nia)