Small change to keep the library compatible with latest JUCE

[PatrickKunz]

The library does not compile anymore with the latest juce version because of unique pointers:

sfzero::Sound *sound = dynamic_cast<sfzero::Sound *>(getSound(0));

needs to be changed to:

sfzero::Sound *sound = dynamic_cast<sfzero::Sound *>(getSound(0).get());

[jyg]

Thanks for the tip. It worked for me. I had also to change occurrences of juce::String::empty into juce::String() in SFZSound.h, SFZReader.cpp, SFZSound.cpp