target: remove double const specificator

Double const is bogus and breaks the build on Clang 3.3.

Change-Id: I9f9394d17f66289ac74ae6dd48f3165483b72e9e
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1489
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: jenkins