libusb: implement libusb_get_parent

Newer versions of drivers such as libwacom (graphics tablets) or
libfprint (fingerprint scanners) call g_usb_device_get_parent. This in
turn calls libusb_get_parent on platforms which implement it, and
returns NULL on platforms that don't. This patch implements this
function on FreeBSD.

Reviewed by: bapt, kevans
Differential Revision: https://reviews.freebsd.org/D46992