ffv1enc: Add const attribute to input frame

warning: assigning to 'AVFrame *' (aka 'struct AVFrame *') from
         'const AVFrame *' (aka 'const struct AVFrame *') discards
         qualifiers [-Wincompatible-pointer-types-discards-qualifiers]

libavcodec/ffv1.h

@@ -78,7 +78,7 @@ typedef struct FFV1Context {
     int transparency;
     int flags;
     int picture_number;
-    AVFrame *frame;
+    const AVFrame *frame;
     AVFrame *last_picture;
 
     AVFrame *cur;