mkhelp.pl: support reproducible build

Do not generate line with the current date, such as:

* Generation time: Tue Oct-24 18:01:41 2017

This will improve reproducibility. The generated string is only
part of a comment, so there should be no adverse consequences.

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>

closes #2026