rdt: Set AVFMT_NOFILE on ff_rdt_demuxer

This makes rdt work again, which has been broken since
603b8bc2a1. This commit made
opening a demuxer without a file (or in this case, with a filename
which can't be opened) fail, unless the demuxer actually declared
AVFMT_NOFILE.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rmdec.c

@@ -972,4 +972,5 @@ AVInputFormat ff_rdt_demuxer = {
     .long_name      = NULL_IF_CONFIG_SMALL("RDT demuxer"),
     .priv_data_size = sizeof(RMDemuxContext),
     .read_close     = rm_read_close,
+    .flags          = AVFMT_NOFILE,
 };