avcodec/ratecontrol: Use forward declaration for AVExpr

Avoids including eval.h everywhere where mpegvideo.h
is included.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/ratecontrol.h

@@ -28,9 +28,7 @@
  * ratecontrol header.
  */
 
-#include <stdio.h>
 #include <stdint.h>
-#include "libavutil/eval.h"
 
 typedef struct Predictor{
     double coeff;
@@ -80,7 +78,7 @@ typedef struct RateControlContext{
     int frame_count[5];
     int last_non_b_pict_type;
 
-    AVExpr * rc_eq_eval;
+    struct AVExpr *rc_eq_eval;
 }RateControlContext;
 
 struct MpegEncContext;