libc: minimal: include: move fcntl.h to posix

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

To ensure a smooth migration, a header was left in
`lib/libc/minimal/include` that prints a deprecation warning.

Users should either include `<zephyr/posix/fcntl.h>` or switch to
`CONFIG_POSIX_API=y`.

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