powerpc build fail

[me@tobin.cc (Tobin C. Harding)]

The current (2bfe01e) torvalds git tree fails to build with the following error

arch/powerpc/kernel/time.c: In function ?running_clock?:
arch/powerpc/kernel/time.c:712:25: error: implicit declaration of function ?cputime_to_nsecs? [-Werror=implicit-function-declaration]
  return local_clock() - cputime_to_nsecs(kcpustat_this_cpu->cpustat[CPUTIME_STEAL]);
                         ^
I would like to do more than just file a bug report. Further
investigation (cputime.h) shows that cputime_to_nsecs has a
preprocessor guard on the config option (currently enabled)
CONFIG_VIRT_CPU_ACCOUNTING_NATIVE. 

CONFIG_PPC_PSERIES is also enabled.

The offending code in arch/powerpc/kernel/time.c is

#ifdef CONFIG_PPC_PSERIES

/*                                                                                    
 * Running clock - attempts to give a view of time passing for a virtualised          
 * kernels.                                                                           
 * Uses the VTB register if available otherwise a next best guess.                    
 */
unsigned long long running_clock(void)
{
 	/*                                                                            
         * Don't read the VTB as a host since KVM does not switch in host             
         * timebase into the VTB when it takes a guest off the CPU, reading the       
         * VTB would result in reading 'last switched out' guest VTB.                 
         *                                                                            
         * Host kernels are often compiled with CONFIG_PPC_PSERIES checked, it        
         * would be unsafe to rely only on the #ifdef above.                          
         */
	if (firmware_has_feature(FW_FEATURE_LPAR) &&
            cpu_has_feature(CPU_FTR_ARCH_207S))
		return mulhdu(get_vtb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;

	/*                                                                            
         * This is a next best approximation without a VTB.                           
         * On a host which is running bare metal there should never be any stolen     
         * time and on a host which doesn't do any virtualisation TB *should* equal   
         * VTB so it makes no difference anyway.                                      
         */
	return local_clock() - cputime_to_nsecs(kcpustat_this_cpu->cpustat[CPUTIME_STEAL]);
}
#endif

Can anyone give me a hint on this one? Where to start reading?

thanks,
Tobin.