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>