[clang-tidy] use override instead of virtual

Found with modernize-use-override

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/mixer/plugins/AlsaMixerPlugin.cxx

@@ -86,7 +86,7 @@ public:
 		:Mixer(alsa_mixer_plugin, _listener),
 		 event_loop(_event_loop) {}
 
-	virtual ~AlsaMixer();
+	~AlsaMixer() override;
 
 	void Configure(const ConfigBlock &block);
 	void Setup();

src/mixer/plugins/HaikuMixerPlugin.cxx

@@ -36,14 +36,14 @@ public:
 		 self(_output) {}
 
 	/* virtual methods from class Mixer */
-	virtual void Open() override {
+	void Open() override {
 	}
 
 	void Close() noexcept override {
 	}
 
-	virtual int GetVolume() override;
-	virtual void SetVolume(unsigned volume) override;
+	int GetVolume() override;
+	void SetVolume(unsigned volume) override;
 };
 
 static Mixer *

src/mixer/plugins/PulseMixerPlugin.cxx

@@ -54,7 +54,7 @@ public:
 	{
 	}
 
-	virtual ~PulseMixer();
+	~PulseMixer() override;
 
 	void Offline();
 	void VolumeCallback(const pa_sink_input_info *i, int eol);