[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>

arch/powerpc/platforms/celleb/setup.c

@@ -49,6 +49,7 @@
 #include <asm/machdep.h>
 #include <asm/cputable.h>
 #include <asm/irq.h>
+#include <asm/time.h>
 #include <asm/spu_priv1.h>
 #include <asm/firmware.h>
 #include <asm/of_platform.h>