minijail: add child PID to log messages

BUG=chromium:248792
TEST=unit tests, manual

Manual test
-----------
- gmerge chromeos-minijail
- reboot
- connect to GoogleGuest
- pkill -STOP wpa_supplicant
- egrep "child process [0-9]+ exited" /var/log/messages

Change-Id: I44923c38f924133ab45700653042c27491d466ba
Reviewed-on: https://gerrit.chromium.org/gerrit/58277
Reviewed-by: Jorge Lucangeli Obes <[email protected]>
Tested-by: mukesh agrawal <[email protected]>
Commit-Queue: mukesh agrawal <[email protected]>
1 file changed