[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/pcm/Mix.hxx

@@ -44,7 +44,7 @@ class PcmDither;
  *
  * @return true on success, false if the format is not supported
  */
-gcc_warn_unused_result
+[[nodiscard]]
 bool
 pcm_mix(PcmDither &dither, void *buffer1, const void *buffer2, size_t size,
 	SampleFormat format, float portion1) noexcept;