beware nvidia driver + make world

[Mike Harding]

There was recently a change to /usr/src/sys/proc.h, and after a
rebuild of the world and kernel and subsequent reboot my system
crashed and took some files with it.  This was because I hadn't
rebuilt the nvidia driver so that it used this new proc.h file.

If you are using the nvidia driver, and do a make world, you may want
to remove and reinstall the nvidia driver port before the reboot so
that this doesn't happen to you.

- Mike H.