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>