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/dumpstats.c b/src/dumpstats.c
index 0ee6641..306fc61 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -3463,7 +3463,7 @@
 			     &sess->si[0],
 			     sess->si[0].state,
 			     sess->si[0].flags,
-			     si_fd(&sess->si[0]),
+			     sess->si[0].conn->t.sock.fd,
 			     sess->si[0].exp ?
 			             tick_is_expired(sess->si[0].exp, now_ms) ? "<PAST>" :
 			                     human_time(TICKS_TO_MS(sess->si[0].exp - now_ms),
@@ -3475,7 +3475,7 @@
 			     &sess->si[1],
 			     sess->si[1].state,
 			     sess->si[1].flags,
-			     si_fd(&sess->si[1]),
+			     sess->si[1].conn->t.sock.fd,
 			     sess->si[1].exp ?
 			             tick_is_expired(sess->si[1].exp, now_ms) ? "<PAST>" :
 			                     human_time(TICKS_TO_MS(sess->si[1].exp - now_ms),
@@ -3704,7 +3704,7 @@
 				     " s0=[%d,%1xh,fd=%d,ex=%s]",
 				     curr_sess->si[0].state,
 				     curr_sess->si[0].flags,
-				     si_fd(&curr_sess->si[0]),
+				     curr_sess->si[0].conn->t.sock.fd,
 				     curr_sess->si[0].exp ?
 				     human_time(TICKS_TO_MS(curr_sess->si[0].exp - now_ms),
 						TICKS_TO_MS(1000)) : "");
@@ -3713,7 +3713,7 @@
 				     " s1=[%d,%1xh,fd=%d,ex=%s]",
 				     curr_sess->si[1].state,
 				     curr_sess->si[1].flags,
-				     si_fd(&curr_sess->si[1]),
+				     curr_sess->si[1].conn->t.sock.fd,
 				     curr_sess->si[1].exp ?
 				     human_time(TICKS_TO_MS(curr_sess->si[1].exp - now_ms),
 						TICKS_TO_MS(1000)) : "");