locking/static_keys: Make verify_keys() static

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150803184748.GA80634@lkp-ib04
Signed-off-by: Ingo Molnar <mingo@kernel.org>