Use ++ instead of += 1 for Counter

gaudi/Gaudi!1136 (merged) removes operator+= from Counter as it leads to the false impression that one could do cnt += 10 except that it would produce the same result as cnt += 1.

This MR replaces the only use of Counter::operator+= in LHCb with ++. The change is backward compatible and can be merged any time, but it is needed before I can merge gaudi/Gaudi!1136 (merged).