mlx5: Add include of <linux/slab.h> because of kzalloc()/kfree() use
mlx5: Add include of <linux/slab.h> because of kzalloc()/kfree() use

On some architectures (for example, arm), we don't end up indirectly
pulling in the declaration of kzalloc() and kfree(), and so building
anything that includes <linux/mlx5/driver.h> breaks.  Fix this by adding
an explicit include to get the declaration.

Reported-by: kbuild test robot <24f7fe9d205c8a9f6ade0c2894e14303ca16087f@intel.com>
Signed-off-by: Roland Dreier <0d270388f2f92757a5de0f4bd891d3b392c44c4f@purestorage.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs