windows: Fix various compiler warnings.

windows: Fix various compiler warnings.

Add LIBUSB_CALL where needed to avoid warnings such as the following:

  In file included from src/hardware/hantek-dso/api.c:34:0:
  src/hardware/hantek-dso/dso.h:212:13:
  note: expected 'libusb_transfer_cb_fn' but argument is of type 'void (*)(struct libusb_transfer *)'
   SR_PRIV int dso_get_channeldata(const struct sr_dev_inst *sdi,
               ^