build issue: change <arduino.h> to <Arduino.h>

I couldn't figure out why I kept getting this error:

fatal error: arduino.h: No such file or directory

Figured it out this morning; it's because the Arduino header file is capitalized. It should be #include <Arduino.h>, not #include <arduino.h>. This issue exists in most of the clank-lz controller-specific header files: dacs.h, step_a4950.h, ucbus_drop.h, osap.h, ts.h, vport_usbserial.h, vport.h, cobs.h, and syserror.h.

@jakeread my guess is you don't get this error because you have a $PATH variable somewhere that points arduino.h to Arduino.h. However, as the code exists now it fails to build on a fresh 20.04/VSCode/PlatformIO install for not-easily-Google-able reasons, so I suggest updating your header files to the Arduino standard.