build: fix NetBSD build warning

Seems NetBSD does like the name reboot, so rename to keep happy.

Change-Id: I60ada9d217c4a8386a8d1ff1c88db7335451794e
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1623


Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>