tmpfiles: fix memory leak in arg_exclude_prefixes

tmpfiles: fix memory leak in arg_exclude_prefixes

When using the `--image` or `-E` options, `arg_exclude_prefixes` is extended via
the `exclude_default_prefixes` function, which calls `strv_extend_strv`, adding
values using `strdup` that must be freed on exit.

Also changing `arg_include_prefixes` to use the same model, although there is no
leak here.