PCI: Declare pci_filp_private only when HAVE_PCI_MMAP

PCI: Declare pci_filp_private only when HAVE_PCI_MMAP

The struct pci_filp_private has no users outside drivers/pci/proc.c and is
only used when HAVE_PCI_MMAP is defined.

Wrap the struct pci_filp_private definition itself in #ifdef HAVE_PCI_MMAP.

Found by cppcheck:

  $ cppcheck --enable=all --force drivers/pci/proc.c
  drivers/pci/proc.c:192:6: style: struct member 'pci_filp_private::write_combine' is never used. [unusedStructMember]

Link: https://lore.kernel.org/r/20210706003145.3054881-1-kw@linux.com
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>