Fix CTimeVal definition for platforms where time_t isn't CLong

One such platform is Debian unstable armhf, which is in the process of
transitioning to 64-bit time_t: https://wiki.debian.org/ReleaseGoals/64bit-time

On that platform (as well as any other glibc/musl platform), however,
CTimeVal isn't used for anything at all because there are #ifdefs that
prefer using `utimensat` which takes CTimeSpec instead. This fix is
therefore quite theoretical, as it is unknown whether there are any
platforms actually affected.

Related: https://github.com/haskell/unix/pull/252