fix(compress): gate MT frame callback by build feature

The MT frame-progression callback is only referenced when ZSTD_MULTITHREAD is enabled. Keep its definition under the same configuration guard so single-threaded library, test, and decode-corpus builds do not report an unused static function while the MT callback remains available to the Rust dispatch bridge.

Test Plan:

- git diff --cached --check

- Capped full tests passed before this warning-only guard

- Capped native rebuild and smoke rerun pending

lib/compress/zstd_compress.c

@@ -4439,15 +4439,12 @@ size_t ZSTD_estimateCStreamSize(int compressionLevel)
  * tells how much data has been consumed (input) and produced (output) for current frame.
  * able to count progression inside worker threads (non-blocking mode).
  */
-static ZSTD_frameProgression ZSTD_rust_frameProgression_MT(void* context)
-{
 #ifdef ZSTD_MULTITHREAD
+#  static ZSTD_frameProgression ZSTD_rust_frameProgression_MT(void* context)
+{
     return ZSTDMT_getFrameProgression((ZSTDMT_CCtx*)context);
-#else
-    (void)context;
-    return (ZSTD_frameProgression){ 0 };
-#endif
 }
+#endif
 
 ZSTD_frameProgression ZSTD_getFrameProgression(const ZSTD_CCtx* cctx)
 {