um: virt-pci: fix missing declaration warning

Fix this warning which appears with W=1 and without CONFIG_OF:

 warning: no previous declaration for 'pcibios_get_phb_of_node'
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202308230949.PphIIlhq-lkp@intel.com/
Fixes: 314a1408 ("um: virt-pci: implement pcibios_get_phb_of_node()")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/virt-pci.c

@@ -544,6 +544,7 @@ static void um_pci_irq_vq_cb(struct virtqueue *vq)
 	}
 }
 
+#ifdef CONFIG_OF
 /* Copied from arch/x86/kernel/devicetree.c */
 struct device_node *pcibios_get_phb_of_node(struct pci_bus *bus)
 {
@@ -562,6 +563,7 @@ struct device_node *pcibios_get_phb_of_node(struct pci_bus *bus)
 	}
 	return NULL;
 }
+#endif
 
 static int um_pci_init_vqs(struct um_pci_device *dev)
 {