tools/nolibc: add stdbool.h to nolibc includes

tools/nolibc: add stdbool.h to nolibc includes

Otherwise tests compiled with only "-include nolibc.h" will fail with
"error: unknown type name 'bool'", even though a stdbool.h is available
from nolibc.

Fixes: ae1f550efc11 ("tools/nolibc: add stdbool.h header")
Fixes: f2662ec26b26 ("selftests: kselftest: Create ksft_print_dbg_msg()")
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/lkml/833f5ae5-190e-47ec-9ad9-127ad166c80c@sirena.org.uk/
Signed-off-by: André Almeida <andrealmeid@igalia.com>
[Thomas: add Fixes tags and massage commit message a bit]
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>