[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/DsdiffDecoderPlugin.cxx

@@ -51,7 +51,7 @@ struct DsdiffChunkHeader {
 	 * Read the "size" attribute from the specified header, converting it
 	 * to the host byte order if needed.
 	 */
-	constexpr
+	[[nodiscard]] constexpr
 	uint64_t GetSize() const {
 		return size.Read();
 	}