mm/slab.h: remove duplicate include

Remove duplicate header which is included twice.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/20200818114323.58156-1-yuehaibing@huawei.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>