x86, hw_nmi: Move backtrace_mask declaration under ARCH_HAS_NMI_WATCHDOG

backtrace_mask has been used under the code context of
ARCH_HAS_NMI_WATCHDOG. So put it into that context.
We were warned by the following warning:

  arch/x86/kernel/apic/hw_nmi.c:21: warning: ‘backtrace_mask’ defined but not used
Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
LKML-Reference: <1289573455-3410-2-git-send-email-dzickus@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/apic/hw_nmi.c

@@ -17,15 +17,16 @@
 #include <linux/nmi.h>
 #include <linux/module.h>
 
-/* For reliability, we're prepared to waste bits here. */
-static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
-
 u64 hw_nmi_get_sample_period(void)
 {
 	return (u64)(cpu_khz) * 1000 * 60;
 }
 
 #ifdef ARCH_HAS_NMI_WATCHDOG
+
+/* For reliability, we're prepared to waste bits here. */
+static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
+
 void arch_trigger_all_cpu_backtrace(void)
 {
 	int i;