fix(ppp): Only call peer DNS function if configured

The ppp_set_usepeerdns macro is only defined if both flags are enabled, so
breaks compilation trying to call it if not. Fix is to add the same flags
around the call, so the code now compiles.

components/esp_netif/lwip/esp_netif_lwip_ppp.c

@@ -230,7 +230,11 @@ netif_related_data_t * esp_netif_new_ppp(esp_netif_t *esp_netif, const esp_netif
 #if PPP_NOTIFY_PHASE
     ppp_set_notify_phase_callback(ppp_obj->ppp, on_ppp_notify_phase);
 #endif
+#if PPP_IPV4_SUPPORT
+#if LWIP_DNS
     ppp_set_usepeerdns(ppp_obj->ppp, 1);
+#endif /* LWIP_DNS */
+#endif /* PPP_IPV4_SUPPORT */
 
     return (netif_related_data_t *)ppp_obj;
 }