powerpc/smp: Make debugger_ipi_callback() static

debugger_ipi_callback() is a local routine used as a NMI IPI handler and
does not need to be external.

It fixes this W=1 compile error :

../arch/powerpc/kernel/smp.c:579:6: error: no previous prototype for ‘debugger_ipi_callback’ [-Werror=missing-prototypes]
  579 | void debugger_ipi_callback(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/20210104143206.695198-10-clg@kaod.org