sub: add name field to all sub decoders

Might help with debugging.

sub/sd_lavc_conv.c

@@ -155,6 +155,7 @@ static void uninit(struct sd *sd)
 }
 
 const struct sd_functions sd_lavc_conv = {
+    .name = "lavc_conv",
     .supports_format = supports_format,
     .init = init,
     .decode = decode,