Fix compiler warning for pieusb backend

The debian-10-mini build issues the following warnings:

  CC       libpieusb_la-pieusb_specific.lo
  CC       libpieusb_la-pieusb_usb.lo
In function 'pieusb_initialize_device_definition',
    inlined from 'sanei_pieusb_find_device_callback' at pieusb_specific.c:280:9:
pieusb_specific.c:325:5: warning: 'strncpy' output may be truncated copying 8 bytes from a string of length 8 [-Wstringop-truncation]
     strncpy(buf, inq->vendor, 8);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
pieusb_specific.c:335:5: warning: 'strncpy' output may be truncated copying 16 bytes from a string of length 16 [-Wstringop-truncation]
     strncpy(buf, inq->product, 16);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pieusb_specific.c:349:5: warning: 'strncpy' output may be truncated copying 4 bytes from a string of length 4 [-Wstringop-truncation]
     strncpy(buf, inq->productRevision, 4);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This should be fixed. At first sight, it looks like these should be memcpys instead.