ftdi: make ftdi_location command depend on libusb1 version

ftdi: make ftdi_location command depend on libusb1 version

The function libusb_get_port_numbers(), required for the command
ftdi_location, is only available in recent version of libusb1.
Compilation will break if the function is not available. This patch
enables the command only if libusb1 contains the necessary function.

Change-Id: I091e72dafa4ed22eea51692751d43246a8152987
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3396
Tested-by: jenkins
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>