PCI: Correct the pci_iomap.h header guard #endif comment

Update the include/asm-generic/pci_iomap.h header guard #endif comment to
match the corresponding #ifndef.

Link: https://lore.kernel.org/r/20210803123014.2963814-1-Jonathan.Cameron@huawei.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>