posix: options: file_system_r: include sys/util.h header for MIN

Oddly, even though CI passed when the file_system_r change was
merged, now CI has encountered a build error because MIN() was
not defined.

Include `<zephyr/sys/util.h>` to pull in the definition.

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