net: sockets: fix fcntl.h usage

If we are using `CONFIG_ARCH_POSIX`, then include
`<fcntl.h>`. Otherwise, include `<zephyr/posix/fcntl.h>`
since there are no requirements to use `CONFIG_POSIX_API`
internally.

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