Move `Arduino_USBHIDHost.h` inside `src/` subfolder

[aentinger]

This is in concordance with how-to-best write Arduino libraries as well as addressing the following linting error:

ERROR: library.properties includes field item(s) Arduino_USBHIDHost.h not found in library.                  
       See: https://arduino.github.io/arduino-cli/latest/library-specification/#libraryproperties-file-format
       (Rule LP052)                                                                                          

[cristidragomir97]

Fixed by @pennam in #8