applets/mii_selector: Make destructor virtual

Prevents undefined behavior when destroying derived classes through a
base class pointer.

src/core/frontend/applets/mii_selector.h

@@ -29,6 +29,7 @@ struct MiiSelectorData {
 
 class MiiSelector {
 public:
+    virtual ~MiiSelector() = default;
     virtual void Setup(const MiiSelectorConfig& config) {
         this->config = MiiSelectorConfig(config);
     }