[monojob] Release reference on completed job

monojob_wait() was holding a reference to the completed job, meaning that
various objects would not be freed until the next job was plugged in to
the monojob interface.

src/core/monojob.c

 	rc = monojob_rc;
 
 done:
+	job_done ( &monojob, rc );
 	if ( rc ) {
 		printf ( " %s\n", strerror ( rc ) );
 	} else {