nmi_watchdog: Fix undefined 'apic' build bug

Ingo provided me a config that fails to compile with:

  arch/x86/built-in.o: In function
  `arch_trigger_all_cpu_backtrace': (.text+0x17e78): undefined
  reference to `apic' make: *** [.tmp_vmlinux1] Error 1

I realized I changed the compile behaviour of the nmi code by
not wrapping it with CONFIG_LOCAL_APIC.  To fix this I add a
compile check for ARCH_HAS_NMI_WATCHDOG around
arch_trigger_all_cpu_backtrace.
Signed-off-by: NDon Zickus <dzickus@redhat.com>
Cc: a.p.zijlstra@chello.nl
Cc: gorcunov@gmail.com
Cc: aris@redhat.com
LKML-Reference: <1266548212-24243-1-git-send-email-dzickus@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/apic/hw_nmi.c

@@ -92,6 +92,7 @@ u64 hw_nmi_get_sample_period(void)
         return cpu_khz * 1000;
 }
 
+#ifdef ARCH_HAS_NMI_WATCHDOG
 void arch_trigger_all_cpu_backtrace(void)
 {
 	int i;
@@ -108,6 +109,7 @@ void arch_trigger_all_cpu_backtrace(void)
 		mdelay(1);
 	}
 }
+#endif
 
 /* STUB calls to mimic old nmi_watchdog behaviour */
 #if defined(CONFIG_X86_LOCAL_APIC)