block: constify struct part_type part_type

The struct is never modified so it can be const.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230419-const-partition-v3-2-4e14e48be367@weissschuh.net


Signed-off-by: Jens Axboe <axboe@kernel.dk>