-libusb.c: fix build with kernel < 5.0
=== TEST SCRIPT BEGIN ===
#!/bin/bash
git rev-parse base > /dev/null || exit 0
git config --local diff.renamelimit 0
git config --local diff.renames True
git config --local diff.algorithm histogram
./scripts/checkpatch.pl --mailback base..
=== TEST SCRI
USBDEVFS_GET_SPEED is used since version 5.2.0 and
https://gitlab.com/qemu-project/qemu/-/commit/202d69a715a4b1824dcd7ec1683d027ed2bae6d3
resulting in the following build failure with kernel < 5.0:
../hw/usb/host-libusb.c: In function 'usb_host_open':
../hw/usb/host-libusb.c:953:32: error: 'USBDEV
