powerpc/smp: Include tick_broadcast() prototype

It fixes this W=1 compile error :

../arch/powerpc/kernel/smp.c:569:6: error: no previous prototype for ‘tick_broadcast’ [-Werror=missing-prototypes]
  569 | void tick_broadcast(const struct cpumask *mask)
      |      ^~~~~~~~~~~~~~

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-9-clg@kaod.org