modplug: mark soundsource final

This silences a compiler warning about the virtual close()
function call from the destructor bypassing the virtual dispatch.

src/sources/soundsourcemodplug.h

@@ -13,7 +13,7 @@ namespace mixxx {
 // Class for reading tracker files using libmodplug.
 // The whole file is decoded at once and saved
 // in RAM to allow seeking and smooth operation in Mixxx.
-class SoundSourceModPlug : public SoundSource {
+class SoundSourceModPlug final : public SoundSource {
   public:
     static constexpr int kChannelCount = 2;
     static constexpr int kSampleRate = 44100;