Please fix USBD CDC ACM for C++ compatibility

[Nguyen Hoan Hoang]

Please find attached files fixed for USBD_CDC_ACM for compatibility compilation in C++.

app_usbd_cdc_acm_internal.happ_usbd_cdc_acm.h

[Simonr]

Hi Nguyen

I'll report this internally, thanks a lot for the suggestion!

Best regards,

Simon