kernel.bbclass: don't depend on DATETIME

* http://cgit.openembedded.org/openembedded-core/commit/?id=67e77cd948c69798695d5d5f34163ef89339b35a
  moved DATETIME dependency from MODULE_TARBALL_BASE_NAME to MODULE_IMAGE_BASE_NAME
  but without copying that vardepsexclude

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>