[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/decoder/plugins/VorbisDecoderPlugin.cxx

@@ -86,7 +86,7 @@ public:
 		return ::CheckAudioFormat(vi.rate, sample_format, vi.channels);
 	}
 
-	AudioFormat CheckAudioFormat() const {
+	[[nodiscard]] AudioFormat CheckAudioFormat() const {
 		return CheckAudioFormat(vi);
 	}