BUILD: stream_interface: remove si_fd() and its references

si_fd() is not used a lot, and breaks builds on OpenBSD 5.2 which
defines this name for its own purpose. It's easy enough to remove
this one-liner function, so let's do it.
diff --git a/src/frontend.c b/src/frontend.c
index 652718c..a85ccce 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -52,7 +52,7 @@
  */
 int frontend_accept(struct session *s)
 {
-	int cfd = si_fd(&s->si[0]);
+	int cfd = s->si[0].conn->t.sock.fd;
 
 	tv_zero(&s->logs.tv_request);
 	s->logs.t_queue = -1;