powerpc/64s: Fix build failures with CONFIG_NMI_IPI=n

I broke the build when CONFIG_NMI_IPI=n with my recent commit to add
arch_trigger_cpumask_backtrace(), eg:

  stacktrace.c:(.text+0x1b0): undefined reference to `.smp_send_safe_nmi_ipi'

We should rework the CONFIG symbols here in future to avoid these
double barrelled ifdefs but for now they fix the build.

Fixes: 5cc05910f26e ("powerpc/64s: Wire up arch_trigger_cpumask_backtrace()")
Reported-by: Christophe LEROY <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/arch/powerpc/include/asm/nmi.h b/arch/powerpc/include/asm/nmi.h
index 0f571e0..bd9ba8d 100644
--- a/arch/powerpc/include/asm/nmi.h
+++ b/arch/powerpc/include/asm/nmi.h
@@ -8,7 +8,7 @@
 static inline void arch_touch_nmi_watchdog(void) {}
 #endif
 
-#if defined(CONFIG_PPC_BOOK3S_64) && defined(CONFIG_STACKTRACE)
+#if defined(CONFIG_NMI_IPI) && defined(CONFIG_STACKTRACE)
 extern void arch_trigger_cpumask_backtrace(const cpumask_t *mask,
 					   bool exclude_self);
 #define arch_trigger_cpumask_backtrace arch_trigger_cpumask_backtrace