uml: Fix build breakage after slab.h changes

We now have to to include linux/slab.h explicitly for kmalloc &
friends.  Files that build against host headers already get their
prototypes via um_malloc.h, linux/slab.h may even be unavailable.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Tejun Heo <tj@kernel.org>