hw/pci: Add missing include

hw/pci: Add missing include

Noted while refactoring:

      CC      mips-softmmu/hw/mips/gt64xxx_pci.o
    In file included from include/hw/pci-host/gt64xxx.h:2,
                     from hw/mips/gt64xxx_pci.c:30:
    include/hw/pci/pci_bus.h:23:5: error: unknown type name ‘PCIIOMMUFunc’
         PCIIOMMUFunc iommu_fn;
         ^~~~~~~~~~~~
    include/hw/pci/pci_bus.h:27:5: error: unknown type name ‘pci_set_irq_fn’
         pci_set_irq_fn set_irq;
         ^~~~~~~~~~~~~~
    include/hw/pci/pci_bus.h:28:5: error: unknown type name ‘pci_map_irq_fn’
         pci_map_irq_fn map_irq;
         ^~~~~~~~~~~~~~
    include/hw/pci/pci_bus.h:29:5: error: unknown type name ‘pci_route_irq_fn’
         pci_route_irq_fn route_intx_to_irq;
         ^~~~~~~~~~~~~~~~
    include/hw/pci/pci_bus.h:31:24: error: ‘PCI_SLOT_MAX’ undeclared here (not in a function)
         PCIDevice *devices[PCI_SLOT_MAX * PCI_FUNC_MAX];
                            ^~~~~~~~~~~~
    include/hw/pci/pci_bus.h:31:39: error: ‘PCI_FUNC_MAX’ undeclared here (not in a function)
         PCIDevice *devices[PCI_SLOT_MAX * PCI_FUNC_MAX];
                                           ^~~~~~~~~~~~
    make[1]: *** [rules.mak:69: hw/mips/gt64xxx_pci.o] Error 1
    make: *** [Makefile:482: subdir-mips-softmmu] Error 2

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>