decoder/ffmpeg: pass AVSEEK_FLAG_ANY to av_seek_frame()

This corrects a major mistake from commit 724a59aa - there was one
small thing that commit was supposed to do, and it failed.
AV_TIME_BASE is not a seek flag.

src/decoder/FfmpegDecoderPlugin.cxx

@@ -496,7 +496,7 @@ ffmpeg_decode(Decoder &decoder, InputStream &input)
 				av_stream->start_time;
 
 			if (av_seek_frame(format_context, audio_stream, where,
-					  AV_TIME_BASE) < 0)
+					  AVSEEK_FLAG_ANY) < 0)
 				decoder_seek_error(decoder);
 			else {
 				avcodec_flush_buffers(codec_context);