powerpc/powermac: Add missing prototype for note_bootable_part()

Add a missing prototype for function `note_bootable_part` to silence a
warning treated as error with W=1:

  arch/powerpc/platforms/powermac/setup.c:361:12: error: no previous prototype for ‘note_bootable_part’ [-Werror=missing-prototypes]
Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powermac/setup.c

@@ -352,6 +352,7 @@ static int pmac_late_init(void)
 }
 machine_late_initcall(powermac, pmac_late_init);
 
+void note_bootable_part(dev_t dev, int part, int goodness);
 /*
  * This is __ref because we check for "initializing" before
  * touching any of the __init sensitive things and "initializing"