fix: use readelf with `--lto-syms` in case of LTO
fix: use readelf with `--lto-syms` in case of LTO

In case LTO was activated, the gnu linker places the symbols in
.gnu.lto sections, which `readelf` prior to `2.36.1` did not understand.
Later versions support the `--lto-syms` option, which is now used for
the `test-symbols` test case.

Fixes: https://github.com/varlink/libvarlink/issues/30

varlink/libvarlink,varlink/libvarlink,varlink/libvarlink,varlink/libvarlink