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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NRoland Dreier <roland@purestorage.com>

include/linux/mlx5/driver.h

@@ -38,8 +38,10 @@
 #include <linux/pci.h>
 #include <linux/spinlock_types.h>
 #include <linux/semaphore.h>
+#include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/radix-tree.h>
+
 #include <linux/mlx5/device.h>
 #include <linux/mlx5/doorbell.h>