Revert "Remove useless Arduino.h include"

This reverts commit 629884d.

Author: facchinm

src/libmodbus/modbus.c

@@ -19,6 +19,7 @@
 #include <unistd.h>
 #endif
 #ifdef ARDUINO
+#include <Arduino.h>
 
 #ifndef DEBUG
 #define printf(...) {}