reboot.h is generated. remove it from distribution.

Reference: /n/atom/patch/applied2013/rmreboot.h2
Date: Sat Sep 21 21:05:06 CES 2013
Signed-off-by: quanstro@quanstro.net

# rm /sys/src/9/bcm/reboot.h
# rm /sys/src/9/kw/reboot.h
# rm /sys/src/9/omap/reboot.h