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

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.