Add (defined(MPBIOS) || NACPICA > 0) check around mp_pci_childdetached().
Fixes the build without MPBIOS or ACPI. Modified patch from Paolo Pisati in
current-users@.

While here:
Remove obsolete <arch/x86/pci/msipic.h> include with its guard.
Add comments for multiple endifs for better readability.

(andvar)