MINOR: init: report the list of optionally available services

It's never easy to guess what services are built in. We currently have
the prometheus exporter in contrib/ which is the only extension for now.
Let's enumerate all available ones just like we do for filterr and pollers.
diff --git a/include/proto/stream.h b/include/proto/stream.h
index a8c2992..71ca849 100644
--- a/include/proto/stream.h
+++ b/include/proto/stream.h
@@ -348,6 +348,7 @@
 }
 
 void service_keywords_register(struct action_kw_list *kw_list);
+void list_services(FILE *out);
 
 #endif /* _PROTO_STREAM_H */