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