[POWERPC] iSeries: Declare iSeries_pci_final_fixup in pci.h

Fixes sparse warning:
arch/powerpc/platforms/iseries/pci.c:169:13: warning: symbol 'iSeries_pci_final_fixup' was not declared. Should it be static?

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>