Fix Windows idle counter calculation

[dbwiddis]

Fixes #646

[codecov-io]

Codecov Report

Merging #647 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #647   +/-   ##
=========================================
  Coverage     83.99%   83.99%           
  Complexity      197      197           
=========================================
  Files            28       28           
  Lines          1456     1456           
  Branches        210      210           
=========================================
  Hits           1223     1223           
  Misses          111      111           
  Partials        122      122

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ea859d4...f511363. Read the comment docs.