Hide more symbols
Apparently `-fvisibility=hidden` is not sufficient to hide some of the
symbols in this library. I've explicitly marked the symbols that were
flagged as being incorrectly exported as "hidden" so hopefully no one
drags them out again.
This is a statically linked library, so the symbols shouldn't need to be
exported for this work work. To ensure that this is the case and that
we're still hitting the overridden API with all of the hidden symbols, I
added a debug log line to the fixed `AsyncTask::waitFuture`
implementation and have verified that we see the compat56 log message
emitted from executables that need the compat56 library.
