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.

Tried different combinations of different devices connected to different hubs and it all seems to work. Tried on the AMD Framework 13. Used an ad-hoc program to test this but I'd eventually like to add a tree view kind of thing to usbconfig like in lsusb.