ruby: Add M5_VAR_USED before variables used only inside assert in garnet2.0.

This removes errors when building gem5.fast

src/mem/ruby/network/garnet2.0/RoutingUnit.cc

@@ -181,7 +181,7 @@ RoutingUnit::outportComputeXY(RouteInfo route,
 {
     PortDirection outport_dirn = "Unknown";
 
-    int num_rows = m_router->get_net_ptr()->getNumRows();
+    int M5_VAR_USED num_rows = m_router->get_net_ptr()->getNumRows();
     int num_cols = m_router->get_net_ptr()->getNumCols();
     assert(num_rows > 0 && num_cols > 0);
 
@@ -238,4 +238,5 @@ RoutingUnit::outportComputeCustom(RouteInfo route,
                                  PortDirection inport_dirn)
 {
     assert(0);
+    return -1;
 }