VMCI: include slab.h into files using kmalloc/kfree

Do not rely on implicit header dependencies as they are known to
break.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Andy King <acking@vmware.com>
Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>