datetime: Enable the datetime module for the emu platform

datetime: Enable the datetime module for the emu platform

Fixes a build failure:

  grub-core/commands/date.c:49: undefined reference to `grub_get_weekday_name'
  grub-core/commands/ls.c:155: undefined reference to `grub_unixtime2datetime'

Bug: https://bugs.gentoo.org/711512

Signed-off-by: Mike Gilbert <floppym@gentoo.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>