LINUX: correct include for ktime_get_coarse_real_ts64()

LINUX: correct include for ktime_get_coarse_real_ts64()

The include for the ktime_get_coarse_real_ts64() autoconf test is
incorrect; ktime_get_coarse_real_ts64() has always been in linux/ktime.h
(via #include timekeeping.h), not linux/time.h.

This autoconf test still ran correctly because the OpenAFS build was
inadvertently picking up ktime.h via the default autoconf include path.
Therefore, this commit is needed only to provide documentation and
clarity to future maintainers.

Introduced as a cut-n-paste error (from the current_kernel_time test)
with commit 3c454b39d04f4886536267c211171dae30dc0344 for Linux 4.20.

Change-Id: I994b03a1700330756216c7feab0121c82d0f3ee4
Reviewed-on: https://gerrit.openafs.org/13437
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>