clone08: Fix another compilation failure.

Including both <sched.h> and <linux/sched.h> will produce large amount
of compilation errors under certain libc versions moreover correct way
of getting the CLONE_* constants defined is to define _GNU_SOURCE before
including the <sched.h> (see /usr/include/bits/sched.h).

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>