Make CPURead/WriteFunc structure 'const'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/prep_pci.c b/hw/prep_pci.c
index 80058b1..2a1d0f9 100644
--- a/hw/prep_pci.c
+++ b/hw/prep_pci.c
@@ -107,13 +107,13 @@
     return val;
 }
 
-static CPUWriteMemoryFunc *PPC_PCIIO_write[] = {
+static CPUWriteMemoryFunc * const PPC_PCIIO_write[] = {
     &PPC_PCIIO_writeb,
     &PPC_PCIIO_writew,
     &PPC_PCIIO_writel,
 };
 
-static CPUReadMemoryFunc *PPC_PCIIO_read[] = {
+static CPUReadMemoryFunc * const PPC_PCIIO_read[] = {
     &PPC_PCIIO_readb,
     &PPC_PCIIO_readw,
     &PPC_PCIIO_readl,