mon/Monitor.cc: init 'timecheck_acks' with '0' in constructor

CID 1019623 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
  uninit_member: Non-static class member "timecheck_acks" is not
  initialized in this constructor nor in any functions that it calls.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>