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 0e2af2a9(x86, hw_nmi: Move backtrace_mask declaration under
ARCH_HAS_NMI_WATCHDOG) addressed this warning, but it was reintroduced
by commit 5f2b0ba4(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>