powerpc/watchdog: Declare soft_nmi_interrupt() prototype

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