guest/init: update reboot() call

The reboot() call that guest/init used was an older version, the
manpage talks about libc4 and libc5.
Update it to the current version exported by glibc by using the right
include file and adjusting the parameter.
This also fixed GCC 5.1.0 compile, because linux/reboot.h misses the
actual prototype.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>