powerpc/pseries/ras: Make init_ras_hotplug_IRQ() static

init_ras_hotplug_IRQ() is a local routine used by a machine init call
and it doesn't need to be external.

It fixes this W=1 compile error:

../arch/powerpc/platforms/pseries/ras.c:125:12: error: no previous prototype for ‘init_ras_hotplug_IRQ’ [-Werror=missing-prototypes]
  125 | int __init init_ras_hotplug_IRQ(void)
      |            ^~~~~~~~~~~~~~~~~~~~

Fixes: c9dccf1d ("powerpc/pseries: Enable RAS hotplug events later")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-5-clg@kaod.org