[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/OpusDecoderPlugin.cxx

@@ -94,7 +94,7 @@ public:
 	/**
 	 * Has DecoderClient::Ready() been called yet?
 	 */
-	bool IsInitialized() const {
+	[[nodiscard]] bool IsInitialized() const {
 		return previous_channels != 0;
 	}