MINOR: filters: Export the name of known filters

It could be useful to know if some filter is declared on a proxy or if it is
enabled on a stream.
diff --git a/src/flt_trace.c b/src/flt_trace.c
index 92a51bf..0ccf1fc 100644
--- a/src/flt_trace.c
+++ b/src/flt_trace.c
@@ -33,6 +33,8 @@
 #include <proto/proto_http.h>
 #include <proto/stream.h>
 
+const char *trace_flt_id = "trace filter";
+
 struct flt_ops trace_ops;
 
 struct trace_config {
@@ -709,6 +711,7 @@
 			pos++;
 		}
 		*cur_arg = pos;
+		fconf->id   = trace_flt_id;
 		fconf->ops  = &trace_ops;
 	}