MIPS: IP27: Fix build error if CONFIG_PCI=y and CONFIG_NUMA disabled.

Then <asm-generic/topology.h> will define cpp macro as default definition
for pcibus_to_node resulting in:

  CC      arch/mips/pci/pci-ip27.o
arch/mips/pci/pci-ip27.c:220:7: error: expected identifier or ‘(’ before ‘void’
arch/mips/pci/pci-ip27.c:220:12: error: expected ‘)’ before ‘(’ token
make[1]: *** [arch/mips/pci/pci-ip27.o] Error 1

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>