fix the arduino build.

tensorflow/lite/micro/kernels/fully_connected.h

@@ -24,7 +24,12 @@ namespace tflite {
 // (reference or optimized) must define this function.
 TfLiteRegistration Register_FULLY_CONNECTED();
 
-#if defined(CMSIS_NN)
+#if defined(CMSIS_NN) || defined(ARDUINO)
+// The Arduino is a special case where we use the CMSIS kernels, but because of
+// the current approach to building for Arduino, we do not support -DCMSIS_NN as
+// part of the Arduino build. As a result, we use defined(ARDUINO) as a way to
+// proxy for defined(CMSIS_NN) for this one special case.
+
 // Returns a TfLiteRegistration struct for cmsis-nn kernel variant that only
 // supports int8.
 TfLiteRegistration Register_FULLY_CONNECTED_INT8();