Fix gcov invocation for header coverage

[alexeckert]

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 99.53%. Comparing base (f2ee40f) to head (b370e43).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #39   +/-   ##
=======================================
  Coverage   99.53%   99.53%           
=======================================
  Files          12       12           
  Lines         642      642           
=======================================
  Hits          639      639           
  Misses          3        3           

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.