Fix compile failure in arch/powerpc/kernel/pci-common.c

This fixes the fallout from the recent powerpc merge (commit
489de302):

   CC      arch/powerpc/kernel/pci-common.o
  arch/powerpc/kernel/pci-common.c:160: error: conflicting types for 'pcibios_add_platform_entries'
  include/linux/pci.h:889: error: previous declaration of 'pcibios_add_platform_entries' was here

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Tested-by: Bret Towe <magnade@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>