selftests/nolibc: test compatibility of nolibc and kernel time types

selftests/nolibc: test compatibility of nolibc and kernel time types

Keeping 'struct timespec' and 'struct __kernel_timespec' compatible
allows the source code to stay simple.

Validate that the types stay compatible.

The test is specific to nolibc and does not compile on other libcs, so
skip it there.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20251220-nolibc-uapi-types-v3-10-c662992f75d7@weissschuh.net