Monitor.cc: fix -Wsign-compare

Fix -Wsign-compare, make 'i' unsigned int.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #72 from dalgaaf/wip-da-comp-sign-unsign

Monitor.cc: fix -Wsign-

Reviewed-by: Sage Weil <sage@inktank.com>