x86: Address 'unused' warning in hw_nmi.c again

arch/x86/kernel/apic/hw_nmi.c:29: warning: backtrace_mask defined but not used

commit 0e2af2a(x86, hw_nmi: Move backtrace_mask declaration under
ARCH_HAS_NMI_WATCHDOG) addressed this warning, but it was reintroduced
by commit 5f2b0ba(x86, nmi_watchdog: Remove the old nmi_watchdog).

Move backtrace_mask into the #ifdef arch_trigger_all_cpu_backtrace
section again.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <AANLkTi=rcc38QzoKa6LFy4m++-p_9=Zt4_kDQE=GeKxf@mail.gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/apic/hw_nmi.c

@@ -24,11 +24,11 @@ u64 hw_nmi_get_sample_period(void)
 }
 #endif
 
+#ifdef arch_trigger_all_cpu_backtrace
 
 /* For reliability, we're prepared to waste bits here. */
 static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
 
-#ifdef arch_trigger_all_cpu_backtrace
 void arch_trigger_all_cpu_backtrace(void)
 {
 	int i;