This completely removes a timestamp from the build of keyutils,
following the recommendation of:
"Timestamps are best avoided"
Alternately, one could hard-code a specific timestamp (e.g. 2012-04-18),
or patch it to use the SOURCE_DATE_EPOCH environment variable (which
might at least be acceptible to submit upstream), although since
SOURCE_DATE_EPOCH is always 0 in guix, this seems a bit silly to me. :)