hls: do not allow fallback to generic seeking

This makes little sense due to how HLS works, and only causes some
additional annoyances if the HLS read_seek function fails (for example
if it's a live stream). It was most likely unintended.

libavformat/hls.c

@@ -2333,6 +2333,7 @@ AVInputFormat ff_hls_demuxer = {
     .long_name      = NULL_IF_CONFIG_SMALL("Apple HTTP Live Streaming"),
     .priv_class     = &hls_class,
     .priv_data_size = sizeof(HLSContext),
+    .flags          = AVFMT_NOGENSEARCH,
     .read_probe     = hls_probe,
     .read_header    = hls_read_header,
     .read_packet    = hls_read_packet,