Guenter reported [1] that the pci_iounmap() changes remain problematic,
with sparc64 allnoconfig and tinyconfig still not building due to the
header file changes and confusion with the arch-specific pci_iounmap()
implementation.
I'm pretty convinced that sparc should just use GENERIC_IOMAP instead of
doing its own thing, since it turns out that the sparc64 version of
pci_iounmap() is somewhat buggy (see [2]). But in the meantime, this
just fixes the build by avoiding the trivial re-definition of the empty
case.