ERROR: "usb_mon_register" [drivers/usb/mon/usbmon.ko] undefined!

If you are getting this kind error in Linux when recompiling a kernel module, it usually means that your kernel configuration and the actual configuration used to compile your last kernel are out of sync.

This usually gets resolved by recompiling your kernel, and then (if needed) by recompiling your modules again. Of course, you will need to be running the new kernel for the modules to work.