powerpc: Fix SMP build with disabled CPU hotplugging.

Compiling 2.6.33 with SMP enabled and HOTPLUG_CPU disabled gives me the
following link errors:

  LD      init/built-in.o
  LD      .tmp_vmlinux1
arch/powerpc/platforms/built-in.o: In function `.smp_xics_setup_cpu':
smp.c:(.devinit.text+0x88): undefined reference to `.set_cpu_current_state'
smp.c:(.devinit.text+0x94): undefined reference to `.set_default_offline_state'
arch/powerpc/platforms/built-in.o: In function `.smp_pSeries_kick_cpu':
smp.c:(.devinit.text+0x13c): undefined reference to `.set_preferred_offline_state'
smp.c:(.devinit.text+0x148): undefined reference to `.get_cpu_current_state'
smp.c:(.devinit.text+0x1a8): undefined reference to `.get_cpu_current_state'
make: *** [.tmp_vmlinux1] Error 1

The following change fixes that for me and seems to work as expected.

Signed-off-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>