um: virt-pci: fix missing declaration warning

um: virt-pci: fix missing declaration warning

[ Upstream commit 974b808d85abbc03c3914af63d60d5816aabf2ca ]

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: 314a1408b79a ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>