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: 3f02cb2f ("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>