Detailed coverage is broken with Vala

It seems like `gcov` cannot locate generated C sources correctly.

```
[1/1] Generating HTML coverage report.
FAILED: coverage-html
'gcovr' '--html' '--html-details' '-r' '/home/guillaume/Projets/valum' '-o' '/home/guillaume/Projets/valum/build/meson-logs/coverage.html'
Traceback (most recent call last):
File "/home/guillaume/.local/bin/gcovr", line 1970, in
print_html_report(covdata, options.html_details)
File "/home/guillaume/.local/bin/gcovr", line 1473, in print_html_report
INPUT = open(data['FILENAME'], 'r')
IOError: [Errno 2] No such file or directory: 'build/auth-test.c'
ninja: build stopped: subcommand failed.
```

Reproduce the `coverage-html` command shown in the report and inspect how generated C source paths are passed from Meson to `gcovr`. The work is done when detailed HTML coverage completes without failing to open `build/auth-test.c`.

Written by the indexing model from the issue text.