Use _STRUCT_TIMESPEC as guard in <bits/types/struct_timespec.h> [BZ #23349]

After commit d76d3703 ("Fix missing
timespec definition for sys/stat.h (BZ #21371)") in combination with
kernel UAPI changes, GCC sanitizer builds start to fail due to a
conflicting definition of struct timespec in <linux/time.h>.  Use
_STRUCT_TIMESPEC as the header file inclusion guard, which is already
checked in the kernel header, to support including <linux/time.h> and
<sys/stat.h> in the same translation unit.

(cherry picked from commit c1c2848b)