swim3: add missing major.h include

swim3 got this through blkdev.h previously, but blkdev.h is not including
it anymore. Include it specifically for the driver, otherwise FLOPPY_MAJOR
is undefined and breaks the compile on PPC if swim3 is configured.

Fixes: b81e0c23 ("block: drop unused includes in <linux/genhd.h>")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Jens Axboe <axboe@kernel.dk>