rcu: remove duplicated include in kernel/rcupreempt.c

Removed duplicated include file <linux/rcupdate.h> in kernel/rcupreempt.c.

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>