rcu: Don't include <linux/ktime.h> in rcutiny.h

The kbuild reported a built failure due to a header loop when RCUTINY is
enabled with my pending riscv-nommu port.  Switch rcutiny.h to only
include the minimal required header to get HZ instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>