Do not cast worker->hook function pointers.

I found the function pointer casts fixed in this CL by searching for
"hook = (" and "(AVxWorkerHook)" in the libaom source tree.

BUG=aomedia:2156
BUG=chromium:883004

Change-Id: I1cd705b9d95398986434c0049c80f45dfa996683
diff --git a/av1/encoder/ethread.c b/av1/encoder/ethread.c
index 6917c2f..e8ac30b 100644
--- a/av1/encoder/ethread.c
+++ b/av1/encoder/ethread.c
@@ -27,7 +27,8 @@
   td->rd_counts.skip_mode_used_flag |= td_t->rd_counts.skip_mode_used_flag;
 }
 
-static int enc_worker_hook(EncWorkerData *const thread_data, void *unused) {
+static int enc_worker_hook(void *arg1, void *unused) {
+  EncWorkerData *const thread_data = (EncWorkerData *)arg1;
   AV1_COMP *const cpi = thread_data->cpi;
   const AV1_COMMON *const cm = &cpi->common;
   const int tile_cols = cm->tile_cols;
@@ -241,7 +242,7 @@
   } else {
     num_workers = AOMMIN(num_workers, cpi->num_workers);
   }
-  prepare_enc_workers(cpi, (AVxWorkerHook)enc_worker_hook, num_workers);
+  prepare_enc_workers(cpi, enc_worker_hook, num_workers);
   launch_enc_workers(cpi, num_workers);
   sync_enc_workers(cpi, num_workers);
   accumulate_counters_enc_workers(cpi, num_workers);