[clang-tidy] use override instead of virtual

Found with modernize-use-override

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

src/filter/NullFilter.hxx

@@ -27,7 +27,7 @@ class NullFilter final : public Filter {
 public:
 	explicit NullFilter(const AudioFormat &af):Filter(af) {}
 
-	virtual ConstBuffer<void> FilterPCM(ConstBuffer<void> src) override {
+	ConstBuffer<void> FilterPCM(ConstBuffer<void> src) override {
 		return src;
 	}
 };

src/filter/Observer.cxx

@@ -36,7 +36,7 @@ public:
 		:observer(_observer),
 		 prepared_filter(std::move(_prepared_filter)) {}
 
-	~PreparedProxy() noexcept {
+	~PreparedProxy() noexcept override {
 		assert(child == nullptr);
 		assert(observer.proxy == this);
 
@@ -63,7 +63,7 @@ public:
 		:Filter(_filter->GetOutAudioFormat()),
 		 parent(_parent), filter(std::move(_filter)) {}
 
-	~Proxy() noexcept {
+	~Proxy() noexcept override {
 		parent.Clear(this);
 	}
 

src/filter/plugins/NormalizeFilterPlugin.cxx

@@ -38,7 +38,7 @@ public:
 		:Filter(audio_format), compressor(Compressor_new(0)) {
 	}
 
-	~NormalizeFilter() {
+	~NormalizeFilter() override {
 		Compressor_delete(compressor);
 	}
 

src/filter/plugins/NullFilterPlugin.cxx

@@ -32,7 +32,7 @@
 
 class PreparedNullFilter final : public PreparedFilter {
 public:
-	virtual std::unique_ptr<Filter> Open(AudioFormat &af) override {
+	std::unique_ptr<Filter> Open(AudioFormat &af) override {
 		return std::make_unique<NullFilter>(af);
 	}
 };