selftests/nolibc: drop include guards around standard headers

Nolibc now provides all the headers required by nolibc-test.c.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20250515-nolibc-sys-v1-9-74f82eea3b59@weissschuh.net


Acked-by: Willy Tarreau <w@1wt.eu>