Fixes device_access_usb.c for Win32

[astewart-xmos]

There are two changes in this PR:

• Removes the guard around the #include <stdbool.h> in device_access_usb.c
  • Doesn't build for Win32 without that inclusion. It was clearly excluded from Win32 intentionally, though it isn't obvious to me why.
• Changes find_xmos_device() in device_access_usb.c to return an error, instead of exiting the process
  • This will make the behavior of control_init_usb() for Windows more similar to that of Mac or Linux, which return an error when the device is not found
  • We're using lib_device_control for our VocalSorcery application, and I'm creating a python wrapper for the host library, where I can't have the native code killing the process over a mild exceptional condition.

[larry-xmos]

Thank you for the submission.

I don't remember the details, but c8c6541 suggests that a Windows build failed trying to include stdbool.h. Could there be differences between Visual Studio versions? I would have most likely used 2015 Community.

payload_len_exceeds_control_packet_size uses bool and I expect this on Windows comes windows.h.

I am happy to merge this as VocalFusion uses a tagged version. If we find this breaking Windows builds we can revisit the change on our side.

The find_xmos_device error handling is welcome. Using exit was lazy of me.