aklog build fix: com_err.h header

aklog build fix: com_err.h header

Fix this build error:
aklog_main.c:67:21: error: com_err.h: No such file or directory

On some systems this system header is actually "et/com_err.h".  Cope
by checking for this at configure time and using that location if
needed.

Change-Id: Iaf4fb4fe157ff8e54aa043785377706eab6fb3d1
Reviewed-on: http://gerrit.openafs.org/836
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>