Use the aom_writer type rather than the tag in calling code.

This makes room for typedefing some other struct to aom_writer.

Change-Id: I1e82de1320da00b3e41c90b14f2df45e7628aa89
(cherry picked from commit d69161f8)

av1/encoder/subexp.h

@@ -16,14 +16,12 @@
 extern "C" {
 #endif
 
+#include "aom_dsp/bitwriter.h"
 #include "aom_dsp/prob.h"
 
-struct aom_writer;
+void av1_write_prob_diff_update(aom_writer *w, aom_prob newp, aom_prob oldp);
 
-void av1_write_prob_diff_update(struct aom_writer *w, aom_prob newp,
-                                aom_prob oldp);
-
-void av1_cond_prob_diff_update(struct aom_writer *w, aom_prob *oldp,
+void av1_cond_prob_diff_update(aom_writer *w, aom_prob *oldp,
                                const unsigned int ct[2]);
 
 int av1_prob_diff_update_savings_search(const unsigned int *ct, aom_prob oldp,