add exception that is not needed

trex-gui is trying to fetch this exception and will not compile if removed

Author: Emil-Gustafsson

src/main/java/com/cisco/trex/stateless/TRexClient.java

@@ -599,7 +599,7 @@ private static Stream build1PktSingleBurstStream(Packet pkt) {
         -1);
   }
 
-  public String resolveIpv6(int portIndex, String dstIp) {
+  public String resolveIpv6(int portIndex, String dstIp) throws ServiceModeRequiredException {
     removeRxQueue(portIndex);
     setRxQueue(portIndex, 1000);