selftests/nolibc: prefer <sys/reboot.h> to <linux/reboot.h>

Since both glibc and musl provide RB_ flags via <sys/reboot.h>, and we
just add RB_ flags for nolibc, let's use RB_ flags instead of
LINUX_REBOOT_ flags and only reserve the required <sys/reboot.h> header.

This allows compile libc-test for musl libc without the linux headers.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>