stream_mf: set correct stream origin

mf:// reads files from the local filesystem and should be
subject to the usual security checks mpv has for such protocols.

stream/stream_mf.c

@@ -39,4 +39,5 @@ const stream_info_t stream_info_mf = {
     .name = "mf",
     .open = mf_stream_open,
     .protocols = (const char*const[]){ "mf", NULL },
+    .stream_origin = STREAM_ORIGIN_FS,
 };