Tidy up event hash table definitions a bit more

Tidy up event hash table definitions a bit more

The change in http://gerrit.openafs.org/5314 consolidated the
definitions of struct afs_event and some related objects, but
the definitions were not quite in the right place.  In
http://gerrit.openafs.org/5392 this content was moved to more
standard headers, but a comment and #define were missed.
This change moves the comment to be in the same place as the content.

Change-Id: I86831879e9a2afd9e653182dcba4621624c55471
Reviewed-on: http://gerrit.openafs.org/5403
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>