avcodec/snowenc: fix constness of the AVFrame argument in encode_frame()

Some fields of the frame are changed to reflect encoder decissions like
if its a keyframe. It thus cannot be constant.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/snowenc.c

@@ -1550,7 +1550,7 @@ static void calculate_visual_weight(SnowContext *s, Plane *p){
 }
 
 static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
-                        const AVFrame *pict, int *got_packet)
+                        AVFrame *pict, int *got_packet)
 {
     SnowContext *s = avctx->priv_data;
     RangeCoder * const c= &s->c;