[clang-tidy] use nodiscard

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.

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

src/encoder/plugins/FlacEncoderPlugin.cxx

@@ -87,7 +87,7 @@ public:
 	/* virtual methods from class PreparedEncoder */
 	Encoder *Open(AudioFormat &audio_format) override;
 
-	const char *GetMimeType() const noexcept override {
+	[[nodiscard]] const char *GetMimeType() const noexcept override {
 		return  "audio/flac";
 	}
 };