gsettings: Update abicheck.sh script
gcc-9 is including
mangle_path in the symbols of
gsettings/libdconfsettings.so when building with support for gcov
(--coverage). This means that our ignored symbols had to be updated.
Update abicheck.sh so the test suite passes again. Also include a comment to explain the above and how the test works.
Fixes: #60 (closed)