avcodec/aac/aacdec: set keyframe flag in output frames

Don't depend on the generic code setting this.
This is in preparation for a following change.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/aac/aacdec.c

@@ -2196,6 +2196,7 @@ static int aac_decode_er_frame(AVCodecContext *avctx, AVFrame *frame,
 
     ac->frame->nb_samples = samples;
     ac->frame->sample_rate = avctx->sample_rate;
+    ac->frame->flags |= AV_FRAME_FLAG_KEY;
     *got_frame_ptr = 1;
 
     skip_bits_long(gb, get_bits_left(gb));
@@ -2356,6 +2357,7 @@ static int decode_frame_ga(AVCodecContext *avctx, AACDecContext *ac,
     if (samples) {
         ac->frame->nb_samples = samples;
         ac->frame->sample_rate = avctx->sample_rate;
+        ac->frame->flags |= AV_FRAME_FLAG_KEY;
         *got_frame_ptr = 1;
     } else {
         av_frame_unref(ac->frame);