headers: wchar.h: include sys/stat.h instead of declaring things itself

This follows UCRT headers.

Also removed _WSTAT_DEFINED. _INO_T_DEFINED and _DEV_T_DEFINED are kept
because UCRT also defines them.

Signed-off-by: Zhongteng Gui <dragon-archer@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>