avformat/unix: set is_streamed to true

Currently when a Unix Domain Socket is used as input there is a loss
of data when data is consumed from the stream. Setting is_streamed to
true fixes this, since the unix domain socket is now treated like a
consumable stream.

Fixes: #9346
Signed-off-by: dank074 <torresefrain10@gmail.com>
Reviewed-by: Leo Izen <leo.izen@gmail.com>

libavformat/unix.c

@@ -89,6 +89,7 @@ static int unix_open(URLContext *h, const char *filename, int flags)
     }
 
     s->fd = fd;
+    h->is_streamed = 1;
 
     return 0;