silence unused parm warning for worker thread in loop filter

Change-Id: Id51468f99f8970b8795ce2d254344f4b8d7817d0

vp9/decoder/vp9_dthread.c

diff --git a/vp9/decoder/vp9_dthread.c b/vp9/decoder/vp9_dthread.c
index bc6c418..170ccb5 100644
--- a/vp9/decoder/vp9_dthread.c
+++ b/vp9/decoder/vp9_dthread.c
@@ -124,7 +124,7 @@
 static int loop_filter_row_worker(void *arg1, void *arg2) {
   TileWorkerData *const tile_data = (TileWorkerData*)arg1;
   LFWorkerData *const lf_data = &tile_data->lfdata;
-
+  (void) arg2;
   loop_filter_rows_mt(lf_data->frame_buffer, lf_data->cm, lf_data->planes,
                       lf_data->start, lf_data->stop, lf_data->y_only,
                       lf_data->lf_sync, lf_data->num_lf_workers);