sun4M: add include of slab.h for kzalloc

This was being included implicitly via cgroup.h's inclusion of xattr.h
(which has now been removed).

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Tejun Heo <tj@kernel.org>