lib: posix: update option group of `readdir_r()`

`readdir_r()` belongs to the following option group

POSIX_FILE_SYSTEM_R: Thread-Safe File System

Create a new Kconfig `CONFIG_POSIX_FILE_SYSTEM_R` to compile
it.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
3 files changed