Fix missing timespec definition for sys/stat.h (BZ #21371)

As indicated by the bug report, the 'struct timespec' definition
is not defined for '_XOPEN_SOURCE=700' and '_POSIX_C_SOURCE=200112L'.

It is because current code only includes its definition if __USE_ATFILE
is defined and the define is only set with:

  1. _GNU_SOURCE and/or _ATFILE_SOURCE definition.
  2. _POSIX_C_SOURCE >= 200809L

However, the 'st_*' fields in 'struct stat' are defined if __USE_XOPEN2K8.
This patch uses the same logic for 'struct timespec' inclusion.

Tested on x86_64-linux-gnu.

	* io/sys/stat.h: Use __USE_XOPEN2K8 insteaf of __USE_ATFILE for
	struct timespec definition.