aklog: Make dummy write AIX-specific

aklog: Make dummy write AIX-specific

This weird write() call exists to work around some old AIX-specific
bug. The ifdef looks like it is intended to restrict this to pre-5
AIX, but it also turns this on for all non-AIX platforms.

Make this area AIX-specific, to avoid this weird write on other
platforms that have nothing to do with the relevant workaround.

Change-Id: I092bcadb4ecc6277ae01e44e6a957e6bacc0cf2d
Reviewed-on: https://gerrit.openafs.org/14022
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>