Name all external facing structs

Replace all anonymous structs with struct name matching current typedef
minus _t.

Change-Id: I639295e8f1bc4651fffc9286a7dba5171c92ce92
diff --git a/vpx_codec/vpx_decoder.h b/vpx_codec/vpx_decoder.h
index 5e4968d..ab0818f 100644
--- a/vpx_codec/vpx_decoder.h
+++ b/vpx_codec/vpx_decoder.h
@@ -68,7 +68,7 @@
      * stream. Algorithms may extend this structure with data specific
      * to their bitstream by setting the sz member appropriately.
      */
-    typedef struct
+    typedef struct vpx_codec_stream_info
     {
         unsigned int sz;     /**< Size of this structure */
         unsigned int w;      /**< Width (or 0 for unknown/default) */