fix build with Arduino Core for Silicon Labs EFR32 Series 2

src/BuildOpt.h

@@ -363,6 +363,13 @@
   #define RADIOLIB_ARDUINOHAL_PIN_STATUS_CAST         (PinStatus)
   #define RADIOLIB_ARDUINOHAL_INTERRUPT_MODE_CAST     (PinStatus)
 
+#elif defined(ARDUINO_ARCH_SILABS)
+  // Silicon Labs Arduino
+  #define RADIOLIB_PLATFORM                           "Arduino Silicon Labs"
+  #define RADIOLIB_ARDUINOHAL_PIN_MODE_CAST           (PinMode)
+  #define RADIOLIB_ARDUINOHAL_PIN_STATUS_CAST         (PinStatus)
+  #define RADIOLIB_ARDUINOHAL_INTERRUPT_MODE_CAST     (PinStatus)
+
 #else
   // other Arduino platforms not covered by the above list - this may or may not work
   #define RADIOLIB_PLATFORM                           "Unknown Arduino"