sched: Add "const" to is_idle_task() parameter

This patch fixes a build warning in -next due to a const pointer being
passed to is_idle_task().  Because is_idle_task() does not modify anything,
this commit adds the "const" to is_idle_task()'s argument declaration.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>