devel/libusb: ONLY_FOR_PLATFORM=- DragonFly

The old USB headers were removed long ago and I don't think this 3rd party
USB library is functional on DragonFly anymore (even if it were buildable,
which its not).  I'm wondering if this is not also the case for FreeBSD?

Any important software that depends on libusb (SDL2?) will need to
support the base libusb.so in DragonFly (and FreeBSD?).

(marino)