Fix for compiling with esp-idf v3.3.2

Incorporates a patch from @IvanPorro:
espressif#3760 (comment)

I don't know whether this will compile with <3.3.2, as the API has changed.

Author: elliottkember

Diff for: libraries/WiFi/src/ETH.cpp

@@ -193,8 +193,8 @@ IPAddress ETHClass::gatewayIP()
 
 IPAddress ETHClass::dnsIP(uint8_t dns_no)
 {
-    ip_addr_t dns_ip = dns_getserver(dns_no);
-    return IPAddress(dns_ip.u_addr.ip4.addr);
+    const ip_addr_t * dns_ip = dns_getserver(dns_no);
+    return IPAddress(dns_ip->u_addr.ip4.addr);
 }
 
 IPAddress ETHClass::broadcastIP()

Diff for: libraries/WiFi/src/WiFiSTA.cpp

@@ -488,8 +488,8 @@ IPAddress WiFiSTAClass::dnsIP(uint8_t dns_no)
     if(WiFiGenericClass::getMode() == WIFI_MODE_NULL){
         return IPAddress();
     }
-    ip_addr_t dns_ip = dns_getserver(dns_no);
-    return IPAddress(dns_ip.u_addr.ip4.addr);
+    const ip_addr_t * dns_ip = dns_getserver(dns_no);
+    return IPAddress(dns_ip->u_addr.ip4.addr);
 }
 
 /**