Use const cdf with aom_write_cdf().

Change-Id: I0e254b52d3e347a96f38922c3f00993d3e70538f

aom_dsp/bitwriter.h

@@ -183,8 +183,8 @@ static INLINE void aom_write_tree_record(aom_writer *w,
 }
 
 #if CONFIG_EC_MULTISYMBOL
-static INLINE void aom_write_cdf(aom_writer *w, int symb, aom_cdf_prob *cdf,
-                                 int nsymbs) {
+static INLINE void aom_write_cdf(aom_writer *w, int symb,
+                                 const aom_cdf_prob *cdf, int nsymbs) {
 #if CONFIG_ANS
   struct rans_sym s;
   (void)nsymbs;