Fix USB class declaration

[dmadison]

This was defined as "XInput" but it's used as "XInputUSB". It's referenced within the XINPUT class (send/receive) and as extern in the usb_xinput header. This fixes things so the names match, and also fixes issues related to naming your XINPUT object "XInput".

I'm not entirely sure why this still compiled with the wrong declaration. My best guess is that it was optimized away as the class is all inline anyways.