avcodec/libfdk-aacenc: set keyframe in output packets

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>
(cherry picked from commit 99f87251e0)

libavcodec/libfdk-aacenc.c

@@ -549,6 +549,7 @@ static int aac_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     }
 
     avpkt->size     = out_args.numOutBytes;
+    avpkt->flags   |= AV_PKT_FLAG_KEY;
     *got_packet_ptr = 1;
     return 0;
 }