restrict ack's that need tracing

credits to sergey-safarov for noticing

kamailio/default.cfg

@@ -887,7 +887,7 @@ onsend_route {
     }
     
     #!ifdef SIP_TRACE_ROLE
-    if (is_method("ACK")) {
+    if (is_method("ACK") && isflagset(FLAG_SIP_TRACE)) {
         sip_trace();
     }
     #!endif