fix clone prototype

gcc was complaining with:
reboot.c:33: error: conflicting types for ‘clone’
/usr/include/bits/sched.h:83: note: previous declaration of ‘clone’ was here

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>