Fixed a linker error (MSVC2010) caused by undefined IMixPlugin destructor.

-- Pavel Koshevoy

Author: Konstanty

src/libmodplug/sndfile.h

@@ -459,7 +459,7 @@ typedef struct _MODCOMMAND
 class MODPLUG_EXPORT IMixPlugin
 {
 public:
-virtual ~IMixPlugin();
+virtual ~IMixPlugin() {};
 virtual int AddRef() = 0;
 virtual int Release() = 0;
 virtual void SaveAllParameters() = 0;