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 <fengguang.wu@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>