rcu: Remove __read_mostly annotations from rcu_scheduler_active externs

Remove the __read_mostly attributes from the rcu_scheduler_active
extern declarations, because these attributes are ignored for
prototypes and we'd have to include the full <linux/cache.h> header
to gain this functionally pointless attribute defined.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>