Migrate bitwriter to the interface in aom/master

Change-Id: I73d46229f0feea43cbe933e51da997833cce032b
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index fedc263..82f60fa 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -2890,7 +2890,7 @@
   const int have_tiles = tile_cols * tile_rows > 1;
 #endif  // CONFIG_EXT_TILE
 #if CONFIG_ANS
-  BufAnsCoder *buf_ans = &cpi->buf_ans;
+  struct BufAnsCoder *buf_ans = &cpi->buf_ans;
 #endif  // CONFIG_ANS
 
   *max_tile_size = 0;