libusb: add SUPER_PLUS in speed enum.

Summary:
Some application(like usbmuxd) start to use LIBUSB_SPEED_SUPER_PLUS now.
Though we don't have the corresponding infra in kernel right now, it is
harmless to have this enum value in userspace to prevent the compile error

Reviewed By: bapt
Sponsored By: FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50359