Add missing mutex header. Closes #1370

[fspindle]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.63%. Comparing base (0a12cf2) to head (9e24a98).
Report is 3 commits behind head on master.

❗ Current head 9e24a98 differs from pull request most recent head bdc08a2. Consider uploading reports for the commit bdc08a2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1373      +/-   ##
==========================================
- Coverage   45.63%   45.63%   -0.01%     
==========================================
  Files        1069     1069              
  Lines      108080   108080              
  Branches    18710    18709       -1     
==========================================
- Hits        49325    49323       -2     
- Misses      58755    58757       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.