how to link to hrtimers in the kernel

Hi all,

I have modified drivers/usb/core/devio.c to use hrtimer_nanosleep(), and 
have included linux/hrtimer.h with it.

But now I am having trouble linking, I get:
ERROR: "hrtimer_nanosleep" [drivers/usb/core/usbcore.ko] undefined!

How do I properly link to the hrtimer library?

Thanks!
George