Fix bug #54569: Compiler warning in ip4.c: unused parameter dest

src/core/ipv4/ip4.c

@@ -163,6 +163,9 @@ ip4_route(const ip4_addr_t *dest)
   }
 #endif /* LWIP_MULTICAST_TX_OPTIONS */
 
+  /* bug #54569: in case LWIP_SINGLE_NETIF=1 and LWIP_DEBUGF() disabled, the following loop is optimized away */
+  LWIP_UNUSED_ARG(dest);
+
   /* iterate through netifs */
   NETIF_FOREACH(netif) {
     /* is the netif up, does it have a link and a valid address? */