Annotate Fruit's exported symbols

If Fruit is compiled normally, _without_ using `-fvisibility=hidden` and then used in client code compiled with `-fvisibility=hidden` switch, the resulting executable segfaults (this issue was reported by Christoph Duelli).
Ideally, Fruit should support client code that uses `-fvisibility=hidden`.

Once `-fvisibility=hidden` is supported, it might be worth also using it when compiling Fruit itself, potentially reducing the compiled size.