oom: move prototypes to appropriate header file

Move the OOM killer's extern function prototypes to include/linux/oom.h and
include it where necessary.

[clg@fr.ibm.com: build fix]
Cc: Andrea Arcangeli <andrea@suse.de>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/s390/mm/cmm.c

@@ -17,6 +17,7 @@
 #include <linux/ctype.h>
 #include <linux/swap.h>
 #include <linux/kthread.h>
+#include <linux/oom.h>
 
 #include <asm/pgalloc.h>
 #include <asm/uaccess.h>