Align tensorArena in IMU_Classifier to 16 bytes

The TensorFlowLite library checks that the supplied arena memory
is aligned to 16 bytes, and crashes if it is not.  This change
ensures the memory is aligned as expected.

Author: Rainycat

GestureToEmoji/ArduinoSketches/IMU_Classifier/IMU_Classifier.ino

@@ -50,7 +50,7 @@ TfLiteTensor* tflOutputTensor = nullptr;
 // Create a static memory buffer for TFLM, the size may need to
 // be adjusted based on the model you are using
 constexpr int tensorArenaSize = 8 * 1024;
-byte tensorArena[tensorArenaSize];
+byte tensorArena[tensorArenaSize] __attribute__((aligned(16)));
 
 // array to map gesture index to a name
 const char* GESTURES[] = {