arm, pm, vmpressure: add missing slab.h includes

arch/arm/mach-tegra/pm.c, kernel/power/console.c and mm/vmpressure.c
were somehow getting slab.h indirectly through cgroup.h which in turn
was getting it indirectly through xattr.h.  A scheduled cgroup change
drops xattr.h inclusion from cgroup.h and breaks compilation of these
three files.  Add explicit slab.h includes to the three files.

A pending cgroup patch depends on this change and it'd be great if
this can be routed through cgroup/for-3.14-fixes branch.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-tegra@vger.kernel.org
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: linux-pm@vger.kernel.org
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: cgroups@vger.kernel.org