oom: prevent including sched.h in header file

It's not necessary to include all of linux/sched.h in linux/oom.h.  Instead,
simply include prototypes for the relevant structs and include linux/types.h
for gfp_t.

Cc: Andrea Arcangeli <andrea@suse.de>
Cc: Christoph Lameter <clameter@sgi.com>
Acked-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>