tools: ynl: Initialise fixed headers to 0 in genetlink-legacy

This eliminates the need for e.g. --json '{"dp-ifindex":0}' which is not
too big a deal for ovs but will get tiresome for fixed header structs that
have many members.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>