MINOR: protocol: add the control layer type in the protocol struct

This one will be needed to more accurately select a protocol. It may
differ from the socket type for QUIC, which uses dgram at the socket
layer and provides stream at the control layer. The upper level requests
a control layer only so we need this field.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index 0dd03f8..6cf35b7 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -49,6 +49,7 @@
 static struct protocol proto_unix = {
 	.name = "unix_stream",
 	.fam = &proto_fam_unix,
+	.ctrl_type = SOCK_STREAM,
 	.sock_domain = PF_UNIX,
 	.sock_type = SOCK_STREAM,
 	.sock_prot = 0,