mpegvideo: fix compilation after 900321b0

Replace:
s->last_picture_ptr->f.key_frame = 0;
with:
s->last_picture_ptr->key_frame = 0;

This is required since mpegvideo.h:Picture now contains an embedded
AVFrame f, rather than a literal copy of the AVFrame fields.

libavcodec/mpegvideo.c

@@ -1114,7 +1114,7 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             /* Allocate a dummy frame */
             i= ff_find_unused_picture(s, 0);
             s->last_picture_ptr= &s->picture[i];
-            s->last_picture_ptr->key_frame = 0;
+            s->last_picture_ptr->f.key_frame = 0;
             if(ff_alloc_picture(s, s->last_picture_ptr, 0) < 0)
                 return -1;
             ff_thread_report_progress((AVFrame*)s->last_picture_ptr, INT_MAX, 0);
@@ -1124,7 +1124,7 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             /* Allocate a dummy frame */
             i= ff_find_unused_picture(s, 0);
             s->next_picture_ptr= &s->picture[i];
-            s->next_picture_ptr->key_frame = 0;
+            s->next_picture_ptr->f.key_frame = 0;
             if(ff_alloc_picture(s, s->next_picture_ptr, 0) < 0)
                 return -1;
             ff_thread_report_progress((AVFrame*)s->next_picture_ptr, INT_MAX, 0);