audio_core/cubeb_sink: Add override specifier to destructor

CubebSinkStream inherits from a base class with a virtual destructor, so
override can be appended to CubebSinkStream's destructor.

src/audio_core/cubeb_sink.cpp

@@ -46,7 +46,7 @@ public:
         }
     }
 
-    ~CubebSinkStream() {
+    ~CubebSinkStream() override {
         if (!ctx) {
             return;
         }