Fix build failure when libusb dev files are not available and
so usb_blaster is disabled.
Change-Id: I4cda7df689cdb4b62b733cbbab813241cf561e29
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3920
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>