nohz: fix compile warning in tick_nohz_init()

nohz: fix compile warning in tick_nohz_init()

cpu is not used after commit 5b8621a68fdcd2baf1d3b413726f913a5254d46a

Signed-off-by: Li Zhong <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Li Zhong <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Kevin Hilman <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>