Use const cdf with aom_read_symbol().

Change-Id: I6e60d9083da8a2d8f7e182e4f12704eddd170df6
diff --git a/aom_dsp/bitreader.h b/aom_dsp/bitreader.h
index c781c5a..1796819 100644
--- a/aom_dsp/bitreader.h
+++ b/aom_dsp/bitreader.h
@@ -223,7 +223,7 @@
 }
 
 #if CONFIG_EC_MULTISYMBOL
-static INLINE int aom_read_cdf_(aom_reader *r, aom_cdf_prob *cdf,
+static INLINE int aom_read_cdf_(aom_reader *r, const aom_cdf_prob *cdf,
                                 int nsymbs ACCT_STR_PARAM) {
   int ret;
 #if CONFIG_ANS