usb_blaster: maintainer-clean Makefile.in

Change-Id: I120fc13a353c63e12d89698869b30c8e751db074
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1480
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>