BUILD/MINOR: stream: avoid a build warning with threads disabled

gcc 6+ complains about a possible null-deref here due to the test in
objt_server() :

       if (objt_server(s->target))
           HA_ATOMIC_ADD(&objt_server(s->target)->counters.retries, 1);

Let's simply change it to __objt_server(). This can be backported to
1.9 and 1.8.
diff --git a/src/stream.c b/src/stream.c
index df778b1..a58c855 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -793,7 +793,7 @@
 		si->state = SI_ST_REQ;
 	} else {
 		if (objt_server(s->target))
-			HA_ATOMIC_ADD(&objt_server(s->target)->counters.retries, 1);
+			HA_ATOMIC_ADD(&__objt_server(s->target)->counters.retries, 1);
 		HA_ATOMIC_ADD(&s->be->be_counters.retries, 1);
 		si->state = SI_ST_ASS;
 	}