vp9_write_bit_buffer.h add consts to suppress warnings in vs

Change-Id: Iacda473090ae84c798f1d93df8284f34bdf646a5

vp9/encoder/vp9_write_bit_buffer.h

diff --git a/vp9/encoder/vp9_write_bit_buffer.h b/vp9/encoder/vp9_write_bit_buffer.h
index 5958b48..1795e05 100644
--- a/vp9/encoder/vp9_write_bit_buffer.h
+++ b/vp9/encoder/vp9_write_bit_buffer.h
@@ -29,7 +29,7 @@
 }
 
 static void vp9_wb_write_bit(struct vp9_write_bit_buffer *wb, int bit) {
-  const int off = wb->bit_offset;
+  const int off = (int)wb->bit_offset;
   const int p = off / CHAR_BIT;
   const int q = CHAR_BIT - 1 - off % CHAR_BIT;
   if (q == CHAR_BIT -1) {