json: Get rid of casts for "jsmntok_t"

json: Get rid of casts for "jsmntok_t"

With the upstream change having landed that adds a name to the
previously anonymous "jsmntok" typedef, we can now add a forward
declaration for that struct in our code. As a result, we no longer have
to store the "tokens" member of "struct grub_json" as a void pointer but
can instead use the forward declaration, allowing us to get rid of casts
of that field.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>