Some fuzzer workarounds

[Evgeny Vereshchagin]

Hi

> Given that the new fuzz targets seem to just fail to compile with
> ```
> projects/elfutils/fuzz-libdwfl.c:48:10: error: unused variable 'res' [-Werror,-Wunused-variable]
>  Dwarf *res = dwfl_module_getdwarf(mod, &bias);
>         ^
> 1 error generated.
> ```

I've just opened https://github.com/google/oss-fuzz/pull/7408 where the fuzz targets are built with -Werror -Wall -Wextra
among other things.

Thanks,
Evgeny Vereshchagin