x86/apic: Get rid of "warning: 'acpi_ioapic_lock' defined but not used"

kbuild test robot reported this against the -RT tree:

|>> arch/x86/kernel/acpi/boot.c:90:21: warning: 'acpi_ioapic_lock' defined but not used [-Wunused-variable]
|    static DEFINE_MUTEX(acpi_ioapic_lock);
|                        ^
|   include/linux/mutex_rt.h:27:15: note: in definition of macro 'DEFINE_MUTEX'
|     struct mutex mutexname = __MUTEX_INITIALIZER(mutexname)
                  ^~~~~~~~~
which is also true (as in non-used) for !RT but the compiler does not
emit a warning.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20161021084449.32523-1-bigeasy@linutronix.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>