futex: Make should_fail_futex() static

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: kbuild-all@01.org
Cc: tipbuild@zytor.com
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <darren@dvhart.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Brian Silverman <bsilver16384@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>