Fixes this built error:

include/linux/sysctl.h:28: included file 'linux/rcupdate.h' is not exported
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Acked-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>