y2038: include: Move struct __timeval64 definition to a separate file

The struct __timeval64's definition has been moved from ./include/time.h to
./include/struct___timeval64.h.

This change would prevent from polluting other glibc namespaces (when
headers are modified to support 64 bit time on architectures with
__WORDSIZE==32).

Now it is possible to just include definition of this particular structure
when needed.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>