fix(decompress): declare heapmode bridge

Add the existing heap-mode bridge declaration beside the other decompression shim prototypes so the new Rust policy seam remains warning-clean under the native C build.

Test Plan:

- git diff --cached --check

- capped full test suite passed before this warning-only fix; final capped native rebuild follows

lib/decompress/zstd_decompress.c

@@ -137,6 +137,7 @@ void ZSTD_dctx_init_platform(ZSTD_DCtx* dctx);
 void ZSTD_rust_dctx_init_platform(void* bmi2, int dynamic_bmi2);
 size_t ZSTD_rust_dctx_default_max_window_size(void);
 int ZSTD_rust_no_forward_progress_max(void);
+int ZSTD_rust_heapmode(void);
 size_t ZSTD_rust_decompress_stack(void* dst, size_t dstCapacity,
                                   const void* src, size_t srcSize);
 size_t ZSTD_rust_decompress_stack_context(ZSTD_DCtx* dctx,