spapr_pci: Include spapr.h instead of playing games with #error

include/hw/pci-host/spapr.h needs hw/ppc/spapr.h.  It checks whether
its header guard is defined, and errors out if it isn't.

Playing games with some other header's guard symbol is not a good
idea.  Just include the frackin' header already.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
