uuidd: use pid_t type when referring to process id

uuidd: use pid_t type when referring to process id

Earlier use of a variable that holds switch enabling boolean to hold process
id was a little bit strange, and not exactly correct.  An int should be good
enough to hold any pid, but it is better to be precise and use the type that
is meant for the job.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>