Filter gcovr to src/ directory only

The coverage report was including 15,781 lines from system headers and
dependencies instead of just the ~1,500 lines of libhttpserver source.
Use --filter to restrict coverage to src/ directory only.

Author: etr

.github/workflows/verify-build.yml

@@ -668,7 +668,7 @@ jobs:
     - name: Generate coverage report
       run: |
         cd build
-        gcovr --root .. --exclude test/ --xml coverage.xml --xml-pretty --print-summary
+        gcovr --root .. --filter '../src/' --xml coverage.xml --xml-pretty --print-summary
       if: ${{ matrix.os-type == 'ubuntu' && matrix.c-compiler == 'gcc' && matrix.debug == 'debug' && matrix.coverage == 'coverage' && success() }}
 
     - name: Upload coverage to Codecov