[POWERPC] Fix build failure when CONFIG_VIRT_CPU_ACCOUNTING is not defined

Without this patch I get the following build failure
  CC      arch/powerpc/platforms/celleb/setup.o
arch/powerpc/platforms/celleb/setup.c:151: error: 'generic_calibrate_decr' undeclared here (not in a function)

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>