ci: Ignore lcov errors on source missing

When using dtrace some temporary files may be leaked as source files and
this may lead to build issues such as

  geninfo: ERROR: unable to open
    /builds/GNOME/glib/_build/.dtrace-temp.ed1c5ba9.c:
    No such file or directory

AFAIK there's no way to keep these temporary files around, so the only
thing we can do is making lcov less strict about missing files.

We can drop the special option from genhtml since it's using the same
lcovrc file

.gitlab-ci/coverage-docker.sh

@@ -27,7 +27,6 @@ for path in _coverage/*.lcov; do
 done
 
 genhtml \
-    --ignore-errors=source \
     --config-file .lcovrc \
     _coverage/*.lcov \
     -o _coverage/coverage