Define FIRSTPASS_STATS as a struct as well as a type.

This will allow it to be forward declared (both in C and in C++).

Change-Id: I5ce9a2c2990f384b4d28948a1dd2d29c8a9795ad
diff --git a/av1/encoder/firstpass.h b/av1/encoder/firstpass.h
index a35c25e..a872ea6 100644
--- a/av1/encoder/firstpass.h
+++ b/av1/encoder/firstpass.h
@@ -38,7 +38,7 @@
  * normalized to each MB. MV related stats (MVc, MVr, etc.) are normalized to
  * the frame width and height. See function normalize_firstpass_stats.
  */
-typedef struct {
+typedef struct FIRSTPASS_STATS {
   /*!
    * Frame number in display order, if stats are for a single frame.
    * No real meaning for a collection of frames.