core: Fix clang build

Recent changes to the build system that made more warnings be flagged as
errors caused building via clang to break.

Fixes #4795

src/audio_core/time_stretch.h

@@ -5,9 +5,17 @@
 #pragma once
 
 #include <cstddef>
-#include <SoundTouch.h>
 #include "common/common_types.h"
 
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wsign-conversion"
+#endif
+#include <SoundTouch.h>
+#if defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+
 namespace AudioCore {
 
 class TimeStretcher {