patch for libdisk

I have not been able to buildworld  for CURRENT from a FBSD 5.0 RELEASE 
box for a few days here.

The problem was not handling all enumerated cases in a swtich in 
src/lib/libdisk/rules.c.

I solved it by adding a "default:" to the end of the switch.

Patch attatched: [my first patch... tell me if there is something wrong 
with it :)]