fix(compress): correct MT callback declaration

Keep the MT frame-progression callback under the ZSTD_MULTITHREAD preprocessor guard using an ordinary C declaration. The previous warning-cleanup edit accidentally prefixed the declaration with a preprocessor marker, which broke the multithreaded compilation path.

Test Plan:

- git diff --cached --check

- Capped native rebuild failed before this fix at the invalid directive; rerun pending

lib/compress/zstd_compress.c

@@ -4440,7 +4440,7 @@ size_t ZSTD_estimateCStreamSize(int compressionLevel)
  * able to count progression inside worker threads (non-blocking mode).
  */
 #ifdef ZSTD_MULTITHREAD
-#  static ZSTD_frameProgression ZSTD_rust_frameProgression_MT(void* context)
+static ZSTD_frameProgression ZSTD_rust_frameProgression_MT(void* context)
 {
     return ZSTDMT_getFrameProgression((ZSTDMT_CCtx*)context);
 }