PCI/IOV: Add missing prototypes for powerpc pcibios interfaces

Add missing prototypes for:

  resource_size_t pcibios_default_alignment(void);
  int pcibios_sriov_enable(struct pci_dev *pdev, u16 num_vfs);
  int pcibios_sriov_disable(struct pci_dev *pdev);

This fixes the following warnings treated as errors when using W=1:

  arch/powerpc/kernel/pci-common.c:236:17: error: no previous prototype for ‘pcibios_default_alignment’ [-Werror=missing-prototypes]
  arch/powerpc/kernel/pci-common.c:253:5: error: no previous prototype for ‘pcibios_sriov_enable’ [-Werror=missing-prototypes]
  arch/powerpc/kernel/pci-common.c:261:5: error: no previous prototype for ‘pcibios_sriov_disable’ [-Werror=missing-prototypes]

Also, commit 978d2d68 ("PCI: Add pcibios_iov_resource_alignment()
interface") added a new function but the prototype was located in the main
header instead of the CONFIG_PCI_IOV specific section.  Move this function
next to the newly added ones.

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>