Export symbols in `.so` shipped by the wheel

[divyegala]

Why are the symbols hidden? It would make linking to dynamic treelite simpler in cuML.

[chyunsu3]

Back in the days, Treelite was using dmlc-core, so I had to hide the C++ symbols to avoid symbol clashes. Now that Treelite doesn't use dmlc-core, the risk of symbol clashes appear to be low. We can make the C++ symbols public again.