tools: kwbimage: Explicitly set version also for kwbimage v0

For documentation purposes update struct main_hdr_v0 to include information
where version of the image must be stored. For kwbimage v0 it obviously
must be 0. By default all image header memory is initialized to zero,
therefore this change has no functional effect.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>