only use ksz8081 for idf versions later than 4.3 (this fixes compile as component for idf 4.3) (#5599)

esp_eth_phy_new_ksz8081 was added in IDF after version 4.3 and generates a compilation error with IDF-4.3. This change will only use esp_eth_phy_new_ksz8081 for IDF versions after 4.3

Author: liebman

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

@@ -288,7 +288,12 @@ bool ETHClass::begin(uint8_t phy_addr, int power, int mdc, int mdio, eth_phy_typ
             break;
 #endif
         case ETH_PHY_KSZ8081:
+#if ESP_IDF_VERSION > ESP_IDF_VERSION_VAL(4,3,0)
             eth_phy = esp_eth_phy_new_ksz8081(&phy_config);
+#else
+            log_e("unsupported ethernet type 'ETH_PHY_KSZ8081'");
+#endif
+            break;
         default:
             break;
     }