libbladeRF: Support build with libusb-1.0.9

Although its use is not recommended (as later versions introduced USB
3.0-related fixes), the only thing holding back the build of
libusb-1.0.9 was a version check for debug purposes.

If libusb doesn't provide libusb_get_version() (1.0.10+), a dummy
function fills in a dummy string to denote this.