Fix nRF52 builds: guard SerialEthernetInterface with ETHERNET_ENABLED

SerialEthernetInterface.cpp is compiled for every nRF52 target because
PlatformIO builds all .cpp files under src/. It includes
SerialEthernetInterface.h, which unconditionally pulls in
<RAK13800_W5100S.h> -- a library only present in the RAK4631 Ethernet
env's lib_deps. As a result any other nRF52 board (e.g. Heltec T114)
fails to build with 'RAK13800_W5100S.h: No such file or directory'.

Wrap the contents of both files in '#ifdef ETHERNET_ENABLED' so they
compile to empty translation units on non-Ethernet builds. RAK4631
Ethernet envs define ETHERNET_ENABLED and are unaffected.

Fixes #2985

src/helpers/nrf52/SerialEthernetInterface.cpp

@@ -1,3 +1,5 @@
+#ifdef ETHERNET_ENABLED
+
 #include "SerialEthernetInterface.h"
 #include "EthernetMac.h"
 #include <SPI.h>
@@ -262,3 +264,5 @@ bool SerialEthernetInterface::isConnected() const {
 void SerialEthernetInterface::loop() {
   Ethernet.maintain();
 }
+
+#endif // ETHERNET_ENABLED