decoder/ffmpeg: fix build problem with FFmpeg 3.4

Regression by commit a22d1c88d7

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1097

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -467,7 +467,11 @@ FfmpegCheckTag(DecoderClient &client, InputStream *is,
 static bool
 IsSeekable(const AVFormatContext &format_context) noexcept
 {
+#if LIBAVFORMAT_VERSION_INT >= AV_VERSION_INT(58, 6, 100)
 	return (format_context.ctx_flags & AVFMTCTX_UNSEEKABLE) != 0;
+#else
+	return false;
+#endif
 }
 
 static void