Fixed exporting of symbols.

Looks like `-export-symbols-regex` doesn't work correctly with `libtool` and C++ code, so use `visibility` attribute instead to mark symbols as exported. Remaining are a couple of `std::` symbols I can't get rid of, and some debug draw functions used by `sherlock265`.

You can check exported symbols using this command (also contains imported symbols):
```
nm -C -D libde265/.libs/libde265.so
```