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>

include/linux/oom.h

 #ifndef __INCLUDE_LINUX_OOM_H
 #define __INCLUDE_LINUX_OOM_H
 
-#include <linux/sched.h>
-
 /* /proc/<pid>/oom_adj set to -17 protects from the oom-killer */
 #define OOM_DISABLE (-17)
 /* inclusive */
@@ -11,6 +9,11 @@
 
 #ifdef __KERNEL__
 
+#include <linux/types.h>
+
+struct zonelist;
+struct notifier_block;
+
 /*
  * Types of limitations to the nodes from which allocations may occur
  */