gendef: Remove printing __DATE__, to improve build reproducibility

This tool isn't evolving particularly much at the moment, so the
additional value of knowing when a particular version was built
doesn't give much extra value.

Normally, a stripped build of gendef (on Linux) produces a binary
with the exact same hash each time. Using __DATE__ breaks that
reproducibility, if rebuilt on different days. It is possible to
avoid the issue by setting SOURCE_DATE_EPOCH to a fixed value when
building, but in this case, I think the printed date doesn't add much
value, so we could simply remove it. Most other similar tools don't
contain such printouts.

Signed-off-by: Martin Storsjö <martin@martin.st>