powerpc/watchdog: Declare soft_nmi_interrupt() prototype

soft_nmi_interrupt() usage requires PPC_WATCHDOG to be configured.
Check the CONFIG definition to declare the prototype.

It fixes this W=1 compile error :

../arch/powerpc/kernel/watchdog.c:250:6: error: no previous prototype for ‘soft_nmi_interrupt’ [-Werror=missing-prototypes]
  250 | void soft_nmi_interrupt(struct pt_regs *regs)
      |      ^~~~~~~~~~~~~~~~~~
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-18-clg@kaod.org

arch/powerpc/include/asm/nmi.h

@@ -4,6 +4,7 @@
 
 #ifdef CONFIG_PPC_WATCHDOG
 extern void arch_touch_nmi_watchdog(void);
+void soft_nmi_interrupt(struct pt_regs *regs);
 #else
 static inline void arch_touch_nmi_watchdog(void) {}
 #endif

arch/powerpc/kernel/watchdog.c

@@ -27,6 +27,7 @@
 #include <linux/smp.h>
 
 #include <asm/paca.h>
+#include <asm/nmi.h>
 
 /*
  * The powerpc watchdog ensures that each CPU is able to service timers.