Fix ANS build.

Add a stub for the missing tell method.

Change-Id: Ifd9eb7a9d7ad9da15aaa1393d66b1b53d6c7ca65

aom_dsp/bitreader.h

@@ -80,7 +80,11 @@ static INLINE int aom_reader_has_error(aom_reader *r) {
 }
 
 static INLINE ptrdiff_t aom_reader_tell(const aom_reader *r) {
-#if CONFIG_DAALA_EC
+#if CONFIG_ANS
+  (void)r;
+  assert(0 && "aom_reader_tell() is unimplemented for ANS");
+  return 0;
+#elif CONFIG_DAALA_EC
   return aom_daala_reader_tell(r);
 #else
   return aom_dk_reader_tell(r);