libc: minimal: include: move sys/stat.h to posix

The `sys/stat.h` header has never been a part of ISO C so move it
to `zephyr/include/posix/sys/`.

To ensure a smooth migration, leave a stub header in
`lib/libc/minimal/include/sys/` that prints a deprecation warning
suggesting developers either include `<zephyr/posix/sys/stat.h>`
or use `CONFIG_POSIX_API=y`.

Signed-off-by: Chris Friedt <cfriedt@meta.com>