Add some lines to avoid minor compilation errors when using PlatformIO.

[johansmacias]

Add a forward declaration in _examples/common/sensors/RAK1903_Optical_OPT3001/RAK1903_OpticalOPT3001.ino for printError() because it's used in configureSensor() before its definition.

src\main.cpp: In function 'void configureSensor()':
src\main.cpp:29:5: error: 'printError' was not declared in this scope

Add #include <Arduino.h> in _examples/common/sensors/RAK1904_Accelerate_LIS3DH/RAK1904_AccelerateLIS3DH.ino to avoid messages like this:

src\main.cpp: In function 'void lis3dh_read_data()':
src\main.cpp:22:3: error: 'Serial' was not declared in this scope

[beegee-tokyo]

@johansmacias

Thank you.