[media] media/platform/mtk-jpeg: add slab.h to fix build errors

Include <linux/slab.h> to fix these build errors:

../drivers/media/platform/mtk-jpeg/mtk_jpeg_core.c: In function 'mtk_jpeg_open':
../drivers/media/platform/mtk-jpeg/mtk_jpeg_core.c:1017:2: error: implicit declaration of function 'kzalloc' [-Werror=implicit-function-declaration]
  ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
../drivers/media/platform/mtk-jpeg/mtk_jpeg_core.c:1017:6: warning: assignment makes pointer from integer without a cast [enabled by default]
  ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
../drivers/media/platform/mtk-jpeg/mtk_jpeg_core.c:1047:2: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]
  kfree(ctx);

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Ming Hsiu Tsai <minghsiu.tsai@mediatek.com>
Cc: Rick Chang <rick.chang@mediatek.com>
Cc: Bin Liu <bin.liu@mediatek.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>