vfio-ccw: Fix a build error due to missing include of linux/slab.h

Include linux/slab.h to fix a build error due to kfree() being undefined.

Fixes: 3f02cb2fd9 ("vfio-ccw: Wire up the CRW irq and CRW region")
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200703022628.6036-1-sean.j.christopherson@intel.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

drivers/s390/cio/vfio_ccw_chp.c

@@ -8,6 +8,7 @@
  *            Eric Farman <farman@linux.ibm.com>
  */
 
+#include <linux/slab.h>
 #include <linux/vfio.h>
 #include "vfio_ccw_private.h"