rsxx: add slab.h include to dma.c

kbuild test robot says:

tree:   git://git.kernel.dk/linux-block.git

 for-3.9/drivers
head:   1262e24a
commit: 8722ff8c [6/8] block: IBM RamSan 70/80 device driver
config: make ARCH=alpha allyesconfig

All error/warnings:

   drivers/block/rsxx/dma.c: In function 'rsxx_complete_dma':
>> drivers/block/rsxx/dma.c:251:2: error: implicit declaration of function 'kmem_cache_free' [-Werror=implicit-function-declaration]
   drivers/block/rsxx/dma.c: In function 'rsxx_queue_discard':
>> drivers/block/rsxx/dma.c:567:2: error: implicit declaration of function 'kmem_cache_alloc' [-Werror=implicit-function-declaration]
>> drivers/block/rsxx/dma.c:567:6: warning: assignment makes pointer from integer without a cast [enabled by default]
   drivers/block/rsxx/dma.c: In function 'rsxx_queue_dma':
>> drivers/block/rsxx/dma.c:601:6: warning: assignment makes pointer from integer without a cast [enabled by default]
   drivers/block/rsxx/dma.c: In function 'rsxx_dma_init':
>> drivers/block/rsxx/dma.c:985:2: error: implicit declaration of function 'KMEM_CACHE' [-Werror=implicit-function-declaration]
>> drivers/block/rsxx/dma.c:985:29: error: 'rsxx_dma' undeclared (first use in this function)
   drivers/block/rsxx/dma.c:985:29: note: each undeclared identifier is reported only once for each function it appears in
>> drivers/block/rsxx/dma.c:985:39: error: 'SLAB_HWCACHE_ALIGN' undeclared (first use in this function)
   drivers/block/rsxx/dma.c: In function 'rsxx_dma_cleanup':
>> drivers/block/rsxx/dma.c:995:2: error: implicit declaration of function 'kmem_cache_destroy' [-Werror=implicit-function-declaration]
   cc1: some warnings being treated as errors

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